In a skillet cook the onion and 2 tablespoons of the butter over moderately low heat, stirring, until it is very 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 the sauce in the bottom of a well-buttered 3-quart gratin dish at least 2 1/2 inches deep, cover it with a layer of potato slices, overlapping the slices slightly, and cover the potatoes with one third of the onions. Sprinkle the onions with one third 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 mixture, covered with foil, in the middle of a preheated 400°F. oven for 20 minutes. Remove the foil and bake the potato mixture for 30 to 35 minutes more, or until the top is golden and the potatoes are tender.