If you have some leftover lasagna, your first instinct might be to throw it in the microwave to heat it up. If you slice it and fry it up in a pan, you'll upgrade your leftovers.
Serious Eats explains why lasagna doesn't reheat well:
Like macaroni and cheese, once it cools and sits overnight, the pasta absorbs liquid from the sauce. What was once a nice loose casserole with creamy sauce and al dente pasta turns into a solid, semi-dry mass with the texture of overly firm custard and no distinct layers, even after reheating it.
Their trick is to slice it and then heat them up in a pan. The cheese melts and each slice crisps up. Best of all: there's no fighting over the corner pieces here because every slice tastes like a corner piece.
The Best Way to Reheat Lasagna [Serious Eats]