"I do not want the vegetables over the meat, I want them under!"
Translation:Nu vreau legumele peste carne, le vreau sub!
I would use sub here. Dedesubt is more like beneath instead of under.
Note that we can say le vreau sub carne. but with dedesubt you either say le vreau dedesubt. and dedesubt of what is inferred or you would have to say le vreau dedesubtul cărnii. which is not as easy as sub carne.
Similarly instead of peste that can easily be turned into peste carne, we can say deasupra if it is inferred, but again we would have to turn it into deasupra cărnii. Also, deasupra would mean above instead of over.
Semantically, sub is more like right under, and dedesubt can mean somewhere below, maybe but not necessarily right under, and peste is more like right on top instead of deasupra which is somewhere above, maybe but not necessarily right on top.