01 -
Turn your oven on to 375°F (190°C). Smear butter on a 9×13-inch baking dish. Peel the potatoes and slice them as thin as you can (about 3mm). Drop the slices in cold water to keep them from turning brown.
02 -
Combine the cream, milk, and minced garlic in a small pot. Stir them together and gently heat over medium until it just starts to bubble. Take it off the burner.
03 -
Drain the potato slices and layer them in the buttered dish so they overlap a little. Pour the warm cream mixture over the top, making sure it soaks everything. Scatter the Gruyère and Parmesan on top, then sprinkle on some salt and pepper.
04 -
Cover the dish with aluminum foil and bake for about 45 minutes. Remove the foil and bake another 20-30 minutes until the top turns golden brown and the potatoes are fork-tender. Let it rest for 10 minutes before digging in.