Heat the oven to 375°F and grease a 9 x 13-inch baking dish. Slice the potatoes evenly into 1/8-inch rounds, preferably with a mandoline.

Melt the butter in a saucepan over medium heat. Add the minced garlic and cook briefly, just until fragrant.

Whisk in the flour and cook for about 1 minute to form a roux. Gradually whisk in the whole milk and heavy cream.

Cook the sauce, stirring often, for 3–5 minutes, until it thickens enough to lightly coat a spoon. Stir in the salt, pepper, and optional nutmeg.

Remove the sauce from the heat and stir in about half of the cheddar and half of the Parmesan. Do not rinse the sliced potatoes, since their surface starch helps thicken the casserole.

Arrange half of the potatoes in the greased dish, adding half of the thinly sliced onion if using. Pour half of the cheese sauce over the layer, then repeat with the remaining potatoes, onion, and sauce.

Cover the dish tightly with foil and bake for 40–45 minutes. Remove the foil, sprinkle the remaining cheddar and Parmesan over the top, and bake for another 20–30 minutes.

Check the center with a knife to make sure the potatoes are tender and the top is browned. Rest the casserole for 10–15 minutes before serving.