Mix Meatball Ingredients: In a large bowl, combine ground beef, breadcrumbs, milk, egg, diced onion, garlic powder, salt, and pepper. Mix gently until just combined. Form into 24 to 30 meatballs.
Brown the Meatballs: Heat 2 tablespoons butter in a large skillet over medium-high heat. Brown the meatballs in batches for 5 to 7 minutes, turning occasionally. Transfer to a plate.
Cook Noodles: Boil egg noodles according to package directions. Drain and set aside.
Create the Sauce: In the same skillet, melt 3 tablespoons butter over medium heat. Whisk in flour to create a roux for 1 to 2 minutes. Gradually add beef broth while whisking until thickened, about 3 to 4 minutes. Remove from heat and stir in sour cream and Worcestershire sauce. Season with salt and pepper.
Combine: Return meatballs to the sauce. Add cooked noodles and mix until coated. Transfer to a 9×13-inch baking dish and top with mozzarella cheese.
Bake: Preheat oven to 350°F. Bake for 25 to 35 minutes until heated through and cheese is melted.
Serve: Let rest 5 minutes, garnish with fresh parsley, and serve warm.