2 pounds russet or Yukon Gold potatoes, peeled and sliced 1/8 to 1/4-inch thick
2 tablespoons chopped chives, divided, optional
Instructions
Preheat the oven to 350 F.
Generously butter a 1 1/2 to 2-quart baking dish.
In a medium saucepan over medium heat, melt 5 tablespoons of butter. Sauté the onion—if using—until softened. Stir in the flour and cook until well incorporated and bubbly, 1 to 2 minutes. Gradually stir in the milk. Cook, stirring, until the sauce is thickened. Stir in the salt, pepper, and nutmeg, if using. Taste and adjust the seasonings.
Layer half of the potato slices in the prepared baking dish. Spoon half the sauce mixture over the potatoes, then top with the remaining potatoes. Spoon the remaining sauce over all.
Dot with the remaining 1 tablespoon of butter.
Cover with foil and bake for 1 hour.
Remove the foil and bake for 20 minutes longer, until the potatoes are tender and the top is lightly browned.