In a large bowl, combine ground beef, egg, bread crumbs, milk and 1/4 tsp. each salt and pepper. Form into 16 meatballs. Heat a large nonstick skillet over medium-high heat; coat with nonstick cooking spray. Add onion and cook 2 minutes. Combine broth and four; add to skillet. Brint to a simmer. Add mushrooms, meatballs and remaining 1/4 tsp. each salt and pepper. Cook 12 minutes, turning meatballs once, until temperature reaches 160 degrees. Stir in souor cream and dill off heat. Serve with noodles and, if desired, broccoli.