Why This Recipe Works

We wanted irresistibly cheesy mashed potatoes that could be made ahead of time. To start, we sliced 4 pounds of Yukon Gold potatoes (enough to ensure a full casserole dish) and boiled them in well-seasoned water. Once they were completely tender, we drained and mashed them with lots of butter until they were smooth and lump-free. Adding two full sticks of butter in this step prevented the potatoes from becoming gluey as we mashed them. Then we stirred in a hefty pour of half-and-half (heavy cream was just a tad too rich, and milk was not quite rich enough). Next came the cheese. Our tasters loved the nutty depth Gruyère provided but also wanted something with a little more cheesiness. Mozzarella fit the bill: Its mild flavor allowed the Gruyère to sing, but it gave the potatoes a supple, stretchy texture that screamed “cheesy.” When we were ready to bake our casserole, we topped the potatoes with buttery panko bread crumbs, which crisped up into the perfect crunchy foil for the luscious potatoes underneath.

Before You Begin

This recipe can easily be halved: Boil the potatoes in a large saucepan and bake them in an 8-inch square baking dish. If made ahead, the casserole will be slightly more firm and set than a freshly made one; we liked both versions. If you'd like to serve 5 to 6 people, see How to Cut This Recipe in Half below.

Instructions

for the potatoes

Adjust oven rack to upper-middle position and heat oven to 400 degrees. Peel 4 pounds Yukon Gold potatoes. Slice potatoes ½ inch thick. Place potatoes and 2 tablespoons table salt in Dutch oven, add water to cover by 1 inch.

Bring to boil over high heat, then reduce heat to medium and simmer until potatoes are tender and can be easily pierced with paring knife, 18 to 20 minutes.

Drain potatoes and return them to pot. Add 16 tablespoons unsalted butter, cut into 16 pieces, and mash with potato masher until smooth and no lumps remain.

Stir in 1 ½ cups half-and-half, 1 teaspoon pepper, and 1 teaspoon table salt until fully combined.

Stir in 6 ounces shredded Gruyère cheese (1½ cups), 6 ounces shredded whole-milk mozzarella cheese (1½ cups), and ½ cup minced fresh chives until incorporated.

Transfer potato mixture to 13 by 9-inch baking dish and smooth top with spatula.

for the topping

Combine ¾ cup panko bread crumbs, 2 ounces shredded Gruyère cheese (½ cup), and 2 tablespoons melted unsalted butter, in bowl. Sprinkle topping evenly over potato mixture.

Bake until casserole is heated through and topping is crisp and golden brown, about 30 minutes. Let cool for 30 minutes. Serve.TO MAKE AHEAD: At end of step 6, let potato mixture cool completely in dish, cover tightly with plastic wrap, and refrigerate for up to 3 days. When ready to bake, remove plastic, cover dish tightly with aluminum foil, and bake for 25 minutes. Remove foil and proceed with step 7.