Preheat oven to 450°F. In a large bowl, combine beef, bread crumbs, soup mix and milk; shape mixture into 32 meatballs. Place meatballs side by side in a large shallow baking dish, covered with foil. Bake 15 minutes or until meatballs are cooked through. In a medium saucepan, melt butter. Stir in flour and dry mustard, then gradually add milk. Cook over medium heat, stirring constantly, until sauce boils and thickens. Remove from heat and let cool slightly. Add cheese and stir until melted. Stir in meatballs and serve with rice or noodles.