Xennial

Here's what you'll need. It looks like a lot, but of the various options available these are the ones I personally found to be the least stressful to assemble.

THE ABILITY TO LOG IN AT LEAST EVERY 24 HOURS: If you do not water your plants once a day, many of them will die. If you're planting in a Free Company garden you can enlist other members to help. If you're planting in a personal house garden, I believe you can designate someone else a tenant of your house and let them also help? You can't over-water but you can under-water, so the more the merrier.

A HOUSE (FREE COMPANY OR PERSONAL): This can only be done in outdoor gardening plots, which is why Onions are a perk a lot of FCs provide to members. If the only garden you have available is outside a Free Company House, an officer may need to set permissions to give you access to it.

DELUXE GARDEN PATCH (AS MANY AS POSSIBLE): These are the garden plots with 8 spots for seeds/soil, and a scarecrow in the middle. Honestly I'm not sure why they even sell the two smaller size of garden plots, I've never seen anyone use them. Anyway… 2-3 Deluxe Garden Plots make this process easier because you'll usually want to be growing 2-3 different sets of stuff at a time, but if you've only got access to 1 you can still grow Thavnairian Onions, it'll just take longer obviously and you'll have to remember to alternate what you plant each time. A Small House can have 1 Deluxe Garden Patch, a Medium House can have 2, and a Large House can have 3.

FISH MEAL: And a ton of it. Basically any time you're near your garden, go by and sprinkle fish meal on all the seeds. You can sprinkle 1 per real world hour on each seed, and they cut down the time the seed needs to grow (I'm not sure precisely how much, the formula seems to be contentious in the gardening community). Is this 100% necessary? No, you can do without Fish Meal entirely if you want. But the grow times on these seeds are all pretty long, and anything you can do to speed them up will both increase your yield over time and reduce the risk of your seeds drying out if you miss a watering cycle. And honestly, this stuff is cheap enough that it's a very easy call to use it whenever you get the chance. Fish Meal can be bought from housing merchants in every housing district for 21 gil each, or a level 5+ Culinarian can craft 12 of them from 3 Lominsan Anchovies and 1 Fire Shard. And Lominsan Anchovies can be bought for 7 gil each from Holasfhis (Limsa Lominsa Upper Decks X:10.3, Y:13.6) or from the Material Suppliers you can place inside houses (last item under Cooking Ingredients). Or you can fish them yourself from the dock right by the Fishing Guild in Limsa Lominsa, if you're determined to be as cheap as possible about this.

GRADE 3 THANALAN TOPSOIL: You'll need 8 of these per Deluxe Garden Patch you want to fill up. These improve the chance of the seeds that grow in them producing crossbreed seeds, but they still don't guarantee anything. You can acquire Grade 3 Thanalan Topsoil from a Mining node that appears at 5AM Eorzea Time in Western La Noscea in the Hammerlea area (roughly X:18, Y:28), but you can only gather one per spawn of that node. However, once you gather them, if you have a Retainer whose job is set to Miner and is at least level 50 with gear that raises their Gathering score to 149 or higher, you can send them out to gather one Thanalan Topsoil per hour for 2 Ventures. And you can also trade Thanalan Topsoil them from Bertana in Idyllshire (X:5.9, Y:5.2) for one Unidentifiable Ore each. There are a ton of different ways to get Unidentifiable Ores, look it up! The most common is to just buy them for 150 Tomestones of Poetics each from another NPC in the same building as Bertana.

POTTING SOIL: You'll need some of the basic crappy potting soil to optimize your crossbreeding chances, roughly 1 for every 8 Grade 3 Thanalan Topsoil. I'll explain why in a bit. You can just buy these from Material Suppliers/Apartment Merchants in every Housing District for 120 gil each.

MIRROR APPLE SEEDS: You will want 4-5x of these per Deluxe Garden Plot you want to fill. You can get these as a level 42+ Botanist in Coerthas Central Highlands, from the trees immediately to the West of Camp Dragonhead. Being level 55+ for Luck of the Pioneer makes this much easier, as the seeds are usually Hidden, even for max-level Botanists in good gear. You may also want to go and collect your daily Map first, so that you can guarantee that hitting Luck of the Pioneer will make a seed appear instead of a Map that you don't want. Then once you've gathered these at least once, you can send a Botanist Retainer to get another one once per hour. Why do you want 4-5? I'll explain under the crossbreeding section, it'll be up to you to decide if you want to throw away 1 Mirror Apple Seed per plot or one of the other two types of seeds you're pairing it with.

KRAKKA ROOT SEEDS: You'll want 4-5x of these per Deluxe Garden Plot you want to fill. You'll want them in half your available plots usually, or to alternate them if you've got an odd number of plots. You can buy these for 3,480 gil each from the Material Suppliers in the Housing Zones, or from Tanie in New Gridania (X:11.0, Y:11.1). That's a lot of gil when you add up how many you'll eventually go through, so maybe check the market board too. But honestly it's way more likely that people are trying to flip these at a premium than that someone had a bunch of extra to sell at a loss.

OLD WORLD FIG SEEDS: You'll also want 4-5x of these per Delux Garden Plot you want to fill. The other half of your plots that aren't half-full of Krakka Root Seeds will be half-full of Old World Fig Seeds. In either case the other half will be the Mirror Apple Seeds. Old World Fig Seeds can be found from a timed Botany node in The Dravanian Forelands that's available from 2AM-4AM or 2PM-4PM. You can gather at least 8 per spawn of this node, and with Botanist tricks can get as many as 36 pretty easily, so these will probably be among the more abundant ingredients you'll have.

Ok, got all the stuff together? Here's what to do with it!

You're trying to generate Thavnairian Onion Seeds, which only result from successful crossbreeding of other seeds that themselves only emerge from successful crossbreeding. So you want to use your Mirror Apple Seeds and Krakka Root Seeds to crossbreed Curiel Root Seeds, and then use your Mirror Apple Seeds and Old world Fig Seeds to generate Royal Kukuru Seeds. And once you've done that you can use your Curiel Root Seeds and Royal Kukuru Seeds to generate Thavnairian Onion Seeds, which you can either sell or grow.

But how does crossbreeding seeds work?

Basically you want to plant your seeds in alternating patterns so that each seed is next to a different type of seed, and you'll want to use Grade 3 Thanalan Topsoil because it boosts the chances of crossbreeding. Crossbreeding chance is determined at the moment you plant the seeds, so every seed but the first one you plant has a crossbreeding chance based on its neighbor. That's why you plant that first seed using the cheap Potting Soil - the first seed's crossbreed chance will always be zero because it has no neighbors yet, so a Thanalan Topsoil would be wasted on it. Then after you plant the other seven seeds you can dig up the first seed and re-plant another one using the Grade 3 Thanalan Soil, so that now every planted seed has a chance to generate crossbreed seeds.

Once you've planted using that pattern, visit to sprinkle Fish Meal as often as possible (but no more than once per real world hour) and to water the plants at least every 24 hours (and ideally sooner, you don't want to risk cutting it too close).

When you've used this planting pattern to generate at least 4-5 Curiel Root Seeds and 4-5 Royal Kukuru Seeds, you'll want to use the exact same pattern to crossbreed those with each other and generate Thavnairian Onion Seeds. This is where having 3 Garden Plots becomes really ideal, so you can have one plot generating Curiel Root Seeds, one plot generating Royal Kukuru Seeds, and one plot crossbreeding them into Thavnairian Onion Seeds. But realistically even with that setup you'll often have times where the RNG doesn't like you and you don't have enough of either type of tier-2 seed to start making Onion seeds, so don't worry too much if your garden is too small to fully optimize like that.

Thavnairian Onion Seeds have some pretty good crossbreeding potential too, so you may want to put them in a garden outdoors. But you can only get one onion per seed, so fancy soils don't really matter for them unless you're crossbreeding them, and you may decide to plant them indoors so that one or two onion seeds don't disrupt a whole garden plot from generating more onion seeds. That's all a bit beyond the scope of what I want to go into here though, so if you really want to go crazy with more crossbreeding once you've got your onion seeds, you'll have to look it up yourselves.