Cheese, like eggs, should be cooked at low temperatures and never
overcooked.
Melt cheese over simmering water, or add it to a hot mixture. Add
cheese to a sauce after the sauce is cooked and heat only long enough
to melt the cheese. Add cheese to an omelet just before folding.
Bake casseroles containing cheese at low to moderate temperatures. To
keep cheese toppings from toughening or hardening, cover the cheese
with crumbs or add cheese just a few minutes before the casserole comes
out of the oven.
Cheese blends more readily with other ingredients and melts more
quickly if you shred or dice it first.