Remove the foil from the dish and return it to the oven, uncovered, for a further 25 to 35 minutes. During this phase, the cheese on top melts completely and begins to brown and bubble, the edges of the dish take on a beautiful golden color, and the cream sauce reduces slightly and thickens further as some of the liquid evaporates. Watch the dish during this final uncovered phase — if the cheese begins to darken too quickly before the potatoes are completely tender, replace the foil loosely and continue baking. The dish is ready when the potatoes are completely tender all the way through (a knife should slide in with absolutely no resistance), the sauce is thick and creamy rather than watery, and the cheese topping is a deep, uneven golden brown with some darker bubbling spots around the edges.
Step 6: Rest Before Serving
This step is one of the most important in the entire recipe and the one most frequently skipped in the enthusiasm of wanting to eat immediately: allow the potato bake to rest, uncovered, for at least ten to fifteen minutes before cutting into it and serving. During this resting period, the cream sauce continues to thicken as it cools slightly, the layers set and firm up enough to be cut cleanly, and the flavors consolidate and deepen. Cutting into the dish immediately from the oven produces a looser, runnier texture and makes neat portions much harder to achieve. The wait is genuinely worth it.
Tips for the Most Velvety Result
Slice thickness consistency is the most important technical factor in this recipe. If your slices vary significantly in thickness, the dish will not cook evenly. A mandoline set to the three-millimeter position produces perfectly consistent slices in a fraction of the time that hand-slicing requires, and if you make this dish more than once it is very much worth the investment. If slicing by hand, take your time and try to keep a consistent pace and pressure throughout.
The choice of cheese matters more than it might initially seem. Cheddar provides the most familiar and satisfying flavor, while Gruyère and Comté are the traditional French choices for potato gratin and bring a complex, nutty quality that elevates the dish considerably. A combination of cheddar for flavor and mozzarella for stretchy melt gives an excellent result that most people find very appealing. Whatever cheese you choose, grating it fresh from a block rather than using pre-shredded cheese produces a noticeably better melt — pre-shredded cheese is coated with anti-caking agents that prevent clumping but also inhibit the smooth melting that makes a potato bake topping so attractive.
