Spray 3 1/2- to 4-quart slow cooker with cooking spray. In medium bowl, mix turkey, bread crumbs, onion, mustard, salt and pepper. Shape mixture into 16 meatballs. Spray 12-inch nonstick skillet with cooking spray. Add meatballs; cook over medium-high heat until brown. Place meatballs in slow cooker; top with mushrooms. Add broth.