Split the peppers lengthwise and remove seeds. Parboil the peppers in a small amount of boiling salted water for about 5 minutes. Drain well. In a skillet melt 1 tablespoon of the butter and cook the beef and onion, breaking up the meat with a fork, until the meat loses its red color; pour off excess fat. Add 1 teaspoon salt and a dash of pepper. Fill the peppers with the mixture and arrange them in a shallow baking dish. Melt the remaining butter and blend in the flour. Gradually add the milk and cook, stirring, until thickened. Stir in the cheese and season to taste. Pour sauce over the peppers and bake in a preheated 350 oven for about 30 minutes. Makes 4 servings.