In saucepan, combine soup, milk, onion and mustard (can be done in microwave but stir often and watch closely). Cook over medium heat, stirring, until heated through. Remove from heat. Stir in cheese until melted. Pour half of sauce into a buttered 9"x13" baking pan.
Break eggs atop sauce. Spoon the rest of sauce around eggs. Stand bread around edges. Bake at 350 degrees until eggs are set, approximately 30 minutes.