Preheat oven to 350 degrees.

Make the white sauce, first, so it’s ready when the potatoes are cut. The potatoes will oxidize if left and the dish can be too watery if they’re soaked in water.

White Sauce:

For the sauce: In medium saucepan, melt butter.  Stir in flour and cook one to two minutes. Pour in milk, whisking nearly constantly until thickened.

Use the spoon test: when a spoon is dipped in the sauce, turn it over, rounded side up. Run a finger from the handle to the edge of the spoon. If it leaves a track that doesn’t fill in, the sauce has thickened properly. Turn off heat and add salt and pepper, and cheese if using.

Potatoes and Assembly:

Butter a 2 qt casserole dish. Pare potatoes and slice thinly.  Layer potatoes in casserole with onion, and salt and pepper; add another layer of potatoes, sprinkle on the onion, salt and pepper, then top with a layer of potatoes. It’s nice to save enough nicely cut potatoes for the top layer, using the odder cuts in the bottoms layers where they won’t show.

When you get to the top layer, arrange the potatoes neatly, starting around the edge of the dish and slightly over lapping, ending up in the center for a gorgeous presentation.

Pour sauce slowly over potatoes – take a knife and nudge the layers a bit, so the sauce seeps down throughout. Bake, uncovered for one hour to one hour and 20 minutes, until soft and tender all the way through when tested with a knife and lightly browned on top. Let stand several minutes before serving.

Makes 6 one cup servings, or 12 smaller 1/2 cup servings.