Position a rack in the center of the oven and heat the oven to 375°F. Butter a 3-quart (9x13x2-inch) baking dish.

In a 3- to 4-quart saucepan, whisk the cream, milk, garlic, thyme, nutmeg, 2-1/2 tsp. salt, and 1 tsp. pepper. Bring to a simmer over medium heat, stirring occasionally, and then remove from the heat and let cool in the pan to room temperature. Strain through a fine sieve into a large liquid measuring cup.

Arrange about a third of the potatoes in an overlapping layer in the baking dish. Give the cream mixture a quick whisk and pour about a third of it over the potatoes. Repeat twice more with the remaining potatoes and cream mixture. Dot the butter over the top and cover with aluminum foil.

Bake until the potatoes are completely tender when pierced with a paring knife, 45 minutes to 1 hour.

Remove the potatoes from the oven and heat the broiler on high. Uncover the potatoes and gently press them down with a flat spatula so the cream mixture mostly covers them. Broil until nicely browned on top, 5 to 8 minutes. Let the potatoes rest for 5 to 10 minutes before serving.

Make Ahead Tips

The dish can be assembled up to 4 hours ahead, covered, and refrigerated. Let sit at room temperature while the oven heats. You can keep the cooked scalloped potatoes, covered, in a warm oven for up to 1 hour before serving.