I'm not sure that monsters were ever really designed with gastronomy in mind. They don't usually look appetizing.
But, I learned in Nethack that you can convert almost anything into a tasty meal with a high enough cooking skill, so who's to say.... I guess.
The main difference between my world and the real world of say the 13th century is I assume the fantasy world is by virtue of a basically unbroken written history, higher effective technology as the result of magic, and the continual open intervention by numerous activist gods, to be much more cosmopolitian. So, every foodstuff has pretty much migrated pretty much everywhere. Things that in our world were 'new world' foodstuffs are pretty much every already. You can get coffee, tea, potatoes, whatever just about anywhere. This opens up cuisine to the full range of real world possibilities and lets me get away with alot of food anachronisms, but I got to say I've never really considered the possibility of Fillet De Monster.
I guess I've always assumed that monsters generally are either not very appetizing, or else require such extensive preparation before they are safe to consume that the result is a potion and not a meal. About the only thing I can think of that might provoke gastronomic delight are the giant versions of edible species. Honey roasted giant cave cricket legs I can imagine, on the grounds that locusts are edible and insects aren't that far from crabs. But even something like a giant catfish doesn't seem likely to be a delight to me, because I know just how foul a 200 lb mudcat tastes compared to your typical young grain fed farm raised catfish. Octopus is hard and crunchy as it is, without trying to eat a fiberous tough old giant one. With few exceptions (tuna, for example) things that are tasty tend to be things that are small and young. A young spike isn't as impressive on your wall, but it is better eating than a mature 12 point. Mushrooms are best when they are small and the fruit is just forming. Big ones are fiberous and bitter.
A young, dimunitive gelatinous cube might possibly be edible, but it probably doesn't taste any better than say a jellyfish at best, and at worst has all the flavor of a slime mold.
There may well be some fantastic creatures that are good eating, but they are unlikely to be the same ones that present serious challenges to high level adventurers.