It's fundraiser night, so everyone gets the same spaghetti dinner for $10 or whatever. Or maybe certain nights the restaurant part only serves spaghetti. Point is, everyone is eating spaghetti.
The show is going fine. It's a custom written show specifically for spaghetti night, so nobody in the audience has ever seen it before and won't know how it ends. It's a bit of a murder mystery mixed with fantasy or something. The big twist at the end? How was the murderer hiding the bodies? Well, he ground up the bodies to make meatballs, used the blood to thicken up spaghetti sauce, and bought angel hair pasta at Walmart. By the time the twist comes around almost everybody would be on to whatever desert is being served, and it would be pretty funny if it was like ice cream and the spoons they gave you to eat with were shaped like shovels (which was the murder weapon at the beginning of the show).
Getting a bit more into logistics I guess you'd have to make it, like, horror themed to begin with so the patrons don't get too surprised and disgusted by the implication that they just ate a human body (and obviously they're not ACTUALLY eating a human body, it's just normal spaghetti). I guess you'd have to workshop it a bit cus I'd definitely want to go for more of a comedic reveal about the food instead of a drama/horror thing, but it might be a stretch to convince people it's funny that they consumed a human corpse.
