Preheat oven to 425°F (220°C). Spray your largest baking dish (a deep 9x13-inch dish works best) with cooking spray.
In the prepared dish, whisk together the Alfredo sauce, beef broth, Worcestershire sauce, and sour cream until completely smooth.
Stir in the uncooked noodles, onion, meatballs, mushrooms, and garlic. Make sure the noodles are as submerged in the sauce as possible.
Cover tightly with aluminum foil and bake for 30 minutes.
Uncover and stir. Check the noodles. They should be almost al dente. If they're still quite firm, re-cover and bake for another 5 minutes before continuing.
In a small bowl, mix together the breadcrumbs and melted butter.
Sprinkle evenly over the casserole.
Bake, uncovered, for about 5 more minutes until the topping is golden and the noodles are fully cooked. For a crispier topping, place under the broiler for 1-2 minutes.
Let the casserole rest for 5-10 minutes before serving. The sauce will thicken as it sits. Garnish with fresh parsley, if desired.