This casserole is the classic way to make scalloped potatoes and it is delicious. The preparation takes about an hour and the cooking another hour.
2 cups thinly sliced onion
9 Tablespoons butter
6 Tablespoons all-purpose flour
3 ½ cups regular milk, scalded (bring to almost the boiling point) [Use 2% milk if you want a lighter sauce]
2 ½ pounds potatoes, peeled
1 ½ cups coarsely grated sharp Cheddar cheese (~ 6 oz.)
1/3 cup dry bread crumbs
Preheat oven to 400 degrees.
Butter a 3-quart gratin dish that is at least 2 ½ inches deep.
Cook the onion and 2 Tablespoons of the butter in a skillet over moderate low heat. Stir until onions are soft. In a heavy saucepan melt 6 Tablespoons of the remaining butter over moderately low heat, whisk in the flour and cook the roux, whisking, for 3 minutes. Add the milk, scalded, 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 one third of sauce in the bottom of the gratin dish. Cover it with a layer of potato slices, overlapping the slices slight, and cover the potatoes with one third of the onions. Sprinkle the onions with one third of the Cheddar cheese and continue to layer the remaining sauce, potatoes, onions and Cheddar in the same way. Sprinkle the top with bread crumbs and dot it with the remaining 1 Tablespoon butter that is cut into bits. Cover the dish with aluminum foil and bake the scalloped potatoes in the middle of the oven for 20 minutes. Remove the foil and bake for another 30-35 minutes until the top is golden and the potatoes are tender.
~ 8 – 10 servings