Yeah, this happened. Same campaign that the party ran into the Glabrezu at level 2, and yes, they're still alive. How? I have no idea. Anyway.....

I had the idea of a diner of sorts where the chef will take meats from (non-humanoid) creatures, and the chef whips up a meal with it. If it's good, it might get a permanent spot on the menu (this is so they don't have to spend loads of time hunting for foods they like). Their next quest involves going into a swamp full of imps and these poisonous little dragons called Remobra. What should I do if they try to make food out of them? I'm not prepared for the interaction of the party whacking an imp corpse onto the diner counter just yet.

If it's meaty then just make it meaty. If it's something weird like an aberration or a fiend then eating it leaves them poisoned. 

I feel like that gives you a chance to make your own cooking mechanic. Personally I love coming up with stuff like that. What comes top mind is real world puffer fish preparation. Is the chef trained well for "exotic" preparations? Does a character have some skill or item that may help with that? If this going to be a continuing theme then the diner would get better (I'd assume) over time and the more ingredients they have to work with. Unless the party just kept getting shit rolls on, say, preserving the ingredients. Then they could all get sick (poisoned) but maybe the preperation goes very well and if they eat it it's = to a short rest or some other bonus like immune to paralasis for an hour?

Proficiency in cooking tools is a thing. I’d say treat it like any other ability check. Maybe a couple, like a wisdom check to figure out how best to prepare it, then a dex for the execution. And if any of them have proficiency in cooking tools (or of course, the chef feat) let them add their proficiency bonus to the check. 

On another thread I was just writing about an Owlbear Steak Omelette (the steak and eggs are both Owlbear).  Issue 11 of MCDM's Arcadia's last article is Rudy Basso's "I Cut Off It's Snout" which is an article encouraging play with cooking mechanics, and includes the omelette and a number of other recipes.  It's a pretty thoughtful system for maintaining a pantry and obtaining exotic monster ingredients.  Then the recipes are largely of the monstrous type which depending on degree of success in executing the recipe, a consumer can get a range of mechanical/magical benefits for a few hours to the rest of the day.  It's definitely "fun" and probably not compatible with a "serious" game, but if your players want to go there and you're willing to entertain it ... well the mag is like $7 I think (I got grandfathered in at a lower patreon rate), but I feel it's worth it for that article and it's other two articles (one on Angelic ancestries/celestial type lineages, and another giving some good rules and models for fences of the rogue business associate type). 

Also check out the Infernal Rapture Menu in Descent into Avernus.  The only thing that tastes better than chicken is Abyssal Chicken. 

Quote from BTI_Brian >>

I feel like that gives you a chance to make your own cooking mechanic. Personally I love coming up with stuff like that. What comes top mind is real world puffer fish preparation. Is the chef trained well for "exotic" preparations? Does a character have some skill or item that may help with that? If this going to be a continuing theme then the diner would get better (I'd assume) over time and the more ingredients they have to work with. Unless the party just kept getting shit rolls on, say, preserving the ingredients. Then they could all get sick (poisoned) but maybe the preparation goes very well and if they eat it it's = to a short rest or some other bonus like immune to paralysis for an hour?

 Well, I was thinking some of the various foods from creatures I know they can eat might have slight buffs to them. Eldritch Ramen, for example, lets whoever eats it recover some spell slots below 3rd level. I say if the chef can prepare edible dinosaur burgers and scorpion pot-pies, surely they can make something out of an imp. Buffalo Imp Wings, perhaps?

Well Buffalo would be meaningless to most worlds, but definitely Fire Wings.  Actual buffalo sauce Anchor Bar recipe is half butter half hot sauce ... so it does have a sort of infernal decadence to it.  

And your thinking is definitely on track with the article I was mentioning.  Maybe eating Basilisk meat gets you stoneskin for a few hours.  Maybe not the meat, but a broth where Basilisk eyes are the key ingredient.  

The key is, to not just "I eat it and get the bonus" there must be some sort of preparation.  Cooking as a sort of culinary ritual magic even.  Cookbooks becoming a sort of ritual book, etc.  

I think there's definitely good ideas posted and as Midnight pointed out there are some existing mechanics already. I didn't do a search but I bet it would be pretty easy to find some mechanics that other folks have made that you could adapt. Like I said earlier though I like making stuff like that up myself :) I think you're on the track though! Maybe coming up with some ideas yourself of what they might try to prepare. Like the Imp wings and setting certain skills and DC's that reflect the difficulty of preparation. I do a lot of cooking in real life and wings are pretty easy. As long as you have the right cooking temp it's pretty hard to mess up! But what about something like an Imp liver pate? That's a whole 'nother level of skill. First I'd think you'd have to have some kind of check to see if the liver is even still intact after the fight. Then does anyone have the skill to remove it properly and a way to store it until it can be cooked? Then does whoever is preparing it have the skill to cook and flavor it?

Lol, now I'm thinking of all these possibilities :) I'd be happy to help you come up with something. Shoot me a message if you want.

I was thinking about something called the "Battle Burrito". Basically, eating it gives you various combat-based buffs for a short while, but the "Chorizo of Might" as I've called it, the main ingredient, requires five different meats from five different strong creatures. What should the buffs be, and what monsters should make the Chorizo of Might? Any other ingredients that come to mind?

The delicious Basilisk Bisque! Boil Basilisk meat in a hearty milk broth. Add cabbage, corn, and wheat noodles and simmer for 7 1/2 minutes. Eat it and you'll be petrified!

Quote from Gamer_Thirteen >>

The delicious Basilisk Bisque! Boil Basilisk meat in a hearty milk broth. Add cabbage, corn, and wheat noodles and simmer for 7 1/2 minutes. Eat it and you'll be petrified!

 But you'll be full forever!

Quote from SpiderQueenYvonne >>

I was thinking about something called the "Battle Burrito". Basically, eating it gives you various combat-based buffs for a short while, but the "Chorizo of Might" as I've called it, the main ingredient, requires five different meats from five different strong creatures. What should the buffs be, and what monsters should make the Chorizo of Might? Any other ingredients that come to mind?

Chorizo of Might has got to have some Infernal heat to it.  Or maybe the Chorizo is Demonic meat but the sausage is made in Hell.  Yeah, non Devil fiendish meat, but the added heat comes from a Devil's private reserve of pepper oils and hot sauces.  Speaking as someone who knows I guy who had some serious fingernail problems when he didn't take care of himself while making some chorizo from scratch.