8 oz. Gruyère, coarsely grated (about 2 cups), divided, rind reserved if attached
1 (12-oz.) can evaporated milk
2 cups heavy cream
2 garlic cloves, finely grated
1 dried bay leaf
1 sprig rosemary
1 sprig thyme
2 Tbsp. Dijon mustard
1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt
1 tsp. freshly ground pepper
¼ tsp. freshly grated nutmeg
2½ lb. Yukon Gold potatoes (about 7 small)
Step 1
Place a rack in middle of oven and preheat to 375°. Combine Gruyère rind (if using), one 12-oz. can evaporated milk, 2 cups heavy cream, 2 garlic cloves, finely grated, 1 dried bay leaf, 1 rosemary sprig, 1 thyme sprig, 2 Tbsp. Dijon mustard, 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt, 1 tsp. freshly ground pepper, and ¼ tsp. freshly grated nutmeg in a medium Dutch oven or other heavy pot.
Step 2
Peel 2½ lb. Yukon Gold potatoes (about 7 small), then slice into ⅛"-thick rounds with a mandoline or knife, adding directly to cream mixture as you go to prevent oxidation.
Step 3
Place pot over medium heat and bring liquid to a gentle simmer. Immediately reduce heat to medium-low and cook at a bare simmer (do not let it boil), stirring often to help potatoes cook evenly, just until potatoes are fork-tender (you don’t want them to be fully cooked), 6–8 minutes. Remove from heat; fish out herbs and cheese rind and discard.
Step 4
Layer half of the potatoes evenly into an 8x8" glass or ceramic baking dish. Spread ¾ cup cream mixture over, then top evenly with 5 oz. Gruyère, coarsely grated (about 1⅓ cups). Arrange remaining potatoes in an even layer on top and pour another ¾ cup cream mixture over. (You might have some cream mixture leftover in the pot.) Sprinkle remaining 3 oz. Gruyère, coarsely grated (about ⅔ cup) on top.
Step 5
Place baking dish on a rimmed baking sheet and bake until edges are bubbling, top is golden brown, and potatoes are tender, about 45 minutes. Transfer baking dish to a wire rack and let potatoes cool 15 minutes before serving. (If oil has pooled on top, blot away with a paper towel.)