Today, French chefs use a long-simmered, homemade stock to help impart unctuous flavor, deep-brown color, and supreme tenderness to this casserole. We respect their patience, but we think that’s too much time and effort to devote to a side dish.
We wanted a potato casserole with deep flavor and super-tender texture-after a reasonable amount of work.
Since we were seeking a creamy consistency, only one potato variety would do: the moderately starchy, buttery-tasting Yukon Gold. A mandoline was an ideal tool for slicing the peeled spuds thinly; any thicker and the casserole would be too chunky, losing its refined nature.
We added meatiness to our potatoes with some sautéed smoky bacon, which we rendered until crisp and then tossed with the potatoes and some onion we’d sautéed in the same pan as the bacon. We finalized the flavor with a scattering of fresh thyme, sprinkles of salt and pepper, and a few pats of butter.
The best substitute for homemade stock proved to be a combination of chicken and beef broth, which we reduced in the pot we’d used to cook the bacon and onion so that it would pick up all the flavorful bits they had left behind. This step gave the broth a jump start on reducing and, aided by the potatoes’ natural starch, helped ensure that the sauce was the perfect consistency by the time the casserole came out of the oven.
The final key to perfect thickness was allowing the casserole to rest before serving it. This went a long way toward developing a silky, creamy texture, since the starch in the potatoes continued to absorb moisture as they cooled.