Scalloped Potatoes

Steps

1. Heat the oven to 375°F with a rack in the center. Butter a 9 x 13-inch (3-quart) baking dish. Peel the potatoes and keep them in a bowl of cold water while you make the sauce.
2. Melt the 4 tablespoons butter in a wide 6-quart pot over medium heat. Add the onion and cook, stirring, until soft but not browned, 6 to 8 minutes. Stir in the garlic and thyme for 30 seconds.
3. Sprinkle in the flour and stir for 1 minute. Slowly whisk in the milk, then the cream, until smooth. Add the salt, pepper and nutmeg. Bring to a gentle simmer, whisking, and cook until the sauce coats a spoon, about 5 minutes. Take the pot off the heat.
4. Dry the potatoes and slice them 1/8 inch thick with a mandoline or a sharp knife, dropping the slices straight into the sauce and stirring now and then so they stay coated. Do not rinse the slices. Their starch helps thicken the sauce.
5. Set the pot over medium-low heat and bring it to a gentle simmer, stirring gently and scraping the bottom. Cook for 5 to 8 minutes, until the potatoes just start to soften at the edges and the sauce has thickened. Taste the sauce and add more salt if it needs it.
6. Pour everything into the baking dish, spread it level and press the top slices down so the sauce covers them. Set the dish on a rimmed baking sheet to catch any drips, cover tightly with foil and bake for 30 minutes.
7. Take off the foil and bake for 35 to 45 minutes more, until the top is browned in spots, the sauce bubbles thickly at the edges, a knife slides into the center with no resistance and the center reads 165°F.
8. Let it rest for 15 minutes so the sauce can set. Sprinkle with parsley and serve.

Good to know

• Make ahead: bake it up to 1 day before, cool, cover and refrigerate. Reheat it, covered, in a 350°F oven for 30 to 40 minutes, until the center reads 165°F.
• Yukon Golds hold their shape and taste buttery. Russets work too, but they break down more and make a softer dish.
• For a browner top, slide the dish under the broiler for 2 to 3 minutes at the end and watch it closely.