Scalloped potatoes (American creamy potato bake)

Steps

1. Preheat the oven to 190 °C (375 °F) and butter a baking dish of about 2 liters (2 quarts), such as 20 x 28 cm (8 x 11 in).
2. Peel the potatoes and slice them on a mandoline into 3 mm (1/8 in) rounds. Do not rinse them: their starch helps the sauce coat properly. Thinly slice the onion and mince the garlic.
3. Sauce: melt the butter in a saucepan over medium heat, add the onion and cook without browning for 5 minutes, then add the garlic for 30 seconds. Sprinkle in the flour and stir for 1 minute.
4. Pour in the milk in a steady stream while whisking, then add the cream. Stir in the thyme, nutmeg, salt, and pepper, and let simmer for 3 to 4 minutes while whisking, until the sauce thickens slightly and coats the back of a spoon.
5. Arrange one third of the potatoes in overlapping layers in the dish, pour over one third of the sauce, and repeat twice more. Press down gently with a spatula so the top layer is just barely submerged.
6. Cover tightly with aluminum foil and bake for 45 minutes at 190 °C (375 °F), placing it on a baking sheet to catch any drips.
7. Remove the foil and continue baking for 30 to 40 minutes, until the top is golden in spots, the sauce is bubbling around the edges, and a knife tip glides through all the layers with no resistance.
8. Let rest for 15 minutes before serving: the sauce will set and the portions will hold together neatly.