Combine the ingredients. To the base of the slow cooker add the beef broth, french onion soup, mushrooms, Worcestershire sauce, soy sauce, and dijon mustard. Stir. Add the frozen meatballs. Cook on high for 1 hour, or low 4-6 hours.
Mix the cornstarch slurry and add the evaporated milk. In a small bowl, combine the cornstarch and the water. Stir in the evaporated milk then slowly stir in the cornstarch slurry. Cover and cook on high for 1 hour or until sauce starts to thicken.