1. Preheat your oven to 350°F. Grease a large baking dish and set aside.
2. In a large skillet, heat olive oil over medium-high heat. Add the beef and cook until browned, about 5 minutes. Remove from skillet and set aside.
3. In the same skillet, add onions and garlic. Sauté for 3 minutes until onions are translucent, stirring occasionally.
4. Add mushrooms to the skillet and cook for another 5 minutes until they release their juices and begin to brown.
5. Sprinkle flour over the onion and mushroom mixture, stirring constantly for 1-2 minutes to form a roux.
6. Gradually whisk in the beef broth, ensuring there are no lumps. Bring to a simmer and cook until thickened, about 3 minutes.
7. Reduce heat to low, then stir in the sour cream, Dijon mustard, paprika, salt, and pepper. Return the beef to the skillet, combining all ingredients thoroughly.
8. Meanwhile, cook the egg noodles according to package instructions until al dente. Drain and add them to the skillet, mixing well to coat the noodles with the sauce.
9. Transfer the mixture to the prepared baking dish. Top with shredded cheddar cheese.
10. Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
11. Garnish with fresh parsley before serving.

Remember, patience is key! Allow the casserole to rest for a few minutes before serving to let the flavors meld beautifully. This was a tip my Nonna Rosa would always remind me of, ensuring each bite was perfectly seasoned.