In a medium saucepan, melt the butter over low heat. Add the flour, salt and pepper, then use a whisk to blend it in. Gradually stir in the milk. Cook and whisk this mixture over medium-low heat until it is smooth and thickened, about 5 minutes. Remove from the heat. Add the cheese to the sauce pan and mix until smooth. In a lightly greased 1 1/2 quart casserole dish, add the cooked potatoes, green pepper and pimentos. Reserve a few for the top of the casserole if you'd like. Pour the cheese sauce over the top of the potatoes. In a small bowl, combine the bread crumbs and the melted butter. Mix well. Sprinkle the bread crumbs overtop the cheese covered potatoes. If you reserved a few of the green peppers and pimentos, add them to the very top of the casserole. Bake at 325 degrees for 40-45 minutes, or until the casserole is bubbly and heated through.