This is one of my favorite recipes. I make mine in a square 9 inch glass dish and let it crisp on the outside which to me is the best part :)
Ingredients
1/4 cup chopped onion
2 tablespoons butter
2 tablespoons flour
1 1/4 cups milk
2 cups grated Sharp Cheddar Cheese
Salt and pepper to taste
5 medium potatoes, peeled and thinly sliced, not rinsed
Preheat oven to 350 degrees F. Cook onion in butter until soft, add flour and cook for 2 minutes, whisking constantly so it doesn't burn. Whisk in milk and cook until thick and bubbly. Remove from heat and add all but 1/4 cup of cheese, stirring until melted. Layer half of potatoes in a baking dish and sprinkle with salt and pepper. Cover with half of sauce. Layer remaining potato slices on top of sauce and season. Top with remaining sauce and sprinkle with remaining cheese. Bake, covered, for one hour or until potatoes are tender. (I usually take off the foil for the last 15 minutes as I like the top browned also).
This recipe works well with substitutions (low fat milk, skim milk, fat free cheese, and salt substitutes).
