Combine soup, milk, onion, mustard, and cheese. Heat and stir until cheese melts and mixture is smooth. Put about 1 inch of sauce in bottom of a 9x9x2 inch baking dish. Carefully break eggs, so that yolk remains intact, onto sauce. Spoon remaining sauce around and over eggs. Place bread slices around edge of casserole, overlapping slightly. Bake at 350 degrees in a 9x9x2 inch casserole dish for about 20 minutes, or until eggs are done. Serve with ham, bacon, or sausage, if desired.