Without a doubt, this is one of my top treats for the season!

Cook egg noodles: Cook egg noodles according to package directions until al dente. Drain and set aside.
Cook beef: Brown beef sirloin in olive oil over medium-high heat. Season with salt and pepper. Remove from skillet and set aside.
Sauté vegetables: In the same skillet, sauté onion, garlic, and mushrooms until tender.
Make sauce: Sprinkle flour over sautéed vegetables and cook for 1-2 minutes. Gradually add beef broth and heavy cream, stirring constantly. Bring to a simmer and cook until thickened. Stir in sour cream, Parmesan cheese, and mozzarella cheese. Season with salt and pepper.
Combine and assemble: Combine cooked egg noodles, beef, and creamy mushroom sauce in a large bowl. Transfer to a greased 9×13-inch baking dish.
Bake: Bake in preheated oven at 350°F (175°C) for 20-25 minutes, or until bubbly and golden.
Garnish and serve: Sprinkle with fresh parsley and serve hot.