Gratin

The gratin (fr: crust) is a classic of French bourgeois cooking. It is a casserole-style dish that is typically made with some ingredient, and béchamel sauce (sometimes Mornay sauce which is béchamel plus gruyère and parmesan), and/or grated cheese and typically with breadcrumbs on top, baked to a finish with a golden crust.

It should really be regarded as a technique or a meta-recipe rather than a specific recipe even though the words au gratin have entered English as a synonym for au gratin dauphinois (potato gratin.)

This is a sensationally easy dish after you've mastered the art of making béchamel which the CC assures you is really rather easy.