Old-Fashioned Scalloped Potatoes

Ingredients

• 3 lb russet potatoes, peeled
• 5 tbsp unsalted butter
• 1 medium yellow onion, sliced very thin
• 1/3 cup all-purpose flour
• 3 1/2 cups whole milk, warmed
• 2 tsp kosher salt
• 1 tsp black pepper
• 1/4 tsp ground nutmeg
• 2 tbsp chopped fresh chives

Before you cook

• Peel the potatoes and slice them 1/8 inch thick, using a mandoline if you have one.
• Hold the sliced potatoes in a bowl of cold water so they do not brown.
• Slice the onion paper thin.
• Warm the milk until steaming but not boiling.
• Butter a 9-by-13-inch baking dish generously.

Steps

1. Heat the oven to 350 degrees Fahrenheit.
2. Drain the potato slices and pat them dry between clean towels, because wet potatoes will water down the sauce.
3. Melt 3 tablespoons of the butter in a saucepan over medium heat.
4. Add the sliced onion and cook for 6 minutes, until limp and translucent.
5. Whisk in the flour and cook for 2 minutes, stirring constantly, until it smells like toasted bread.
6. Pour in the warm milk in a steady stream while whisking hard.
7. Simmer the sauce for 5 minutes, whisking often, until it thickens enough to coat a spoon.
8. Stir in the salt, pepper, and nutmeg.
9. Lay a third of the potato slices in the buttered dish in overlapping rows.
10. Ladle a third of the sauce over the potatoes and spread it to the corners.
11. Repeat with another third of the potatoes and sauce, then finish with the last of both.
12. Dot the surface with the remaining 2 tablespoons of butter in small pieces.
13. Cover the dish tightly with foil and bake for 50 minutes.
14. Remove the foil and bake for 35 minutes more, until the top is deeply browned and a knife slides through the center without resistance.
15. Let the dish rest for 20 minutes so the sauce sets around the potatoes.
16. Scatter the chives over the top and serve.