You'll love the flavor of this simple gratin. Gruyère is an aged Swiss cheese with a firm texture and sweet, slightly nutty flavor. If you can’t find Gruyère, Comté, Appenzeller, and Emmenthaler are in the same cheese family and good substitutes. This dish is easy to make-ahead. Simply prep the unbaked gratin up to two days ahead and store in the refrigerator. When you're ready to server, bake as directed.