Cook the Noodles: Begin by cooking the wide egg noodles until they're al dente. Drain them well and set them aside for later.
Preheat and Prep: Preheat your oven to 375°F (190°C). Coat a 13x9-inch baking dish with a layer of cooking spray to ensure nothing sticks.
Sauté the Veggies: In a medium saucepan, melt the unsalted butter. Toss in the finely diced onions and thinly sliced mushrooms. Cook these ingredients until they're soft and aromatic, which should take about 3 to 5 minutes.
Create the Roux: Add the all-purpose flour to the vegetable mixture in the saucepan. Whisk continuously to form a smooth roux, cooking it for about a minute.
The Broth Symphony: Slowly pour in the beef broth, blending it with the roux. Add the Worcestershire sauce and let the mixture simmer until it thickens, achieving a luscious consistency.
Creamy Integration: Remove the saucepan from heat. Integrate the sour cream and heavy cream into the sauce. Season your culinary creation with salt and pepper to taste.
Combine and Bake: In a large bowl, amalgamate the cooked noodles, sauce, and thawed meatballs. Transfer this delectable mix to your prepped baking dish. Bake for 20 to 30 minutes, or until the sauce is bubbling and has thickened to perfection.
Finishing Touch: Optionally, sprinkle some freshly chopped parsley over the top of your baked masterpiece for that extra pop of color and flavor.