Melt 4 tablespoons of butter in small saucepan, add flour stirring constantly until well blended and mixture turns light brown. Add milk, nutmeg, and salt and pepper, cook mixture until thickened, stir in cheddar cheese, continue on low heat until cheese is melted and sauce is smooth. In a 2 quart sprayed or buttered pan, layer 1/3 potatoes, 1/3 onion and then top with 1/3 cheese sauce. Continue until all ingredients are used, ending with cheese sauce. Bake at 350 for 40 minutes covered. Uncover and top with breadcrumbs mixed with 2 tablespoons melted butter. Continue baking for 40 more minutes or until potatoes are tender and top is browned.