2 c. thinly sliced onion
9 tbsp. unsalted butter, divided
6 tbsp. flour
3 1/2 c. milk, scalded
2 1/2 lb. boiling potatoes
1 1/2 c. coarsely grated sharp cheddar (about 6 oz.)
1/3 c. dry bread crumbs

In a skillet, cook the onion in 2 tablespoons butter over moderately low heat, stirring until it is very soft.

In a heavy saucepan, melt 6 tablespoons of remaining butter over moderately low heat. Whisk in the flour and cook the Roux whisking for 3 minutes. Add the scalded milk in a stream, whisking and bring the sauce to a boil. Simmer the sauce, whisking for 1 minute and add salt and pepper to taste.

Peel the potatoes and slice them 1/8 inch thick. Spread about 1/3 of the sauce in the bottom of a well buttered 3-quart gratin dish at least 2 1/2 inches deep, over it with a layer of potato slices, overlapping the slices slightly and cover the potatoes with 1/3 of the onions. Sprinkle the onions with 1/3 of the cheddar and continue to layer the remaining sauce, potatoes, onions and cheddar in the same manner.

Sprinkle the top with bread crumbs. Dot it with the remaining 1 tablespoon butter. Cut into bits and bake the potato mixture, covered with foil in the middle of a preheated 400 degree oven for 20 minutes. Remove the foil and bake potato mixture for 30 to 35 minutes more or until the top is golden and the potatoes are tender.

Makes 6-8 servings.