Why parmesan sauce separates in the oven often comes down to prolonged heat or moisture released by the filling. Thicken the sauce before adding Parmesan, drain wet ingredients, and bake just until the casserole is hot.
Essential Concepts
Thicken first. Add cheese over low heat. Limit excess moisture and baking time.
Related posts: [How to Fix Broken Cheese Sauce](/how-to-fix-broken-cheese-sauce/) and [How to Keep Casserole Sauce Creamy](/how-to-keep-casserole-sauce-creamy/).