1. Preheat your oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish with butter or non-stick spray.
2. Heat olive oil or butter in a large skillet over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
3. Add the minced garlic and sliced mushrooms to the skillet. Cook until the mushrooms release their moisture and begin to brown, about 6-8 minutes. Stir occasionally.
4. Push the vegetables to the side and add the ground beef to the skillet. Season with salt, pepper, and dried thyme or Italian seasoning. Brown the meat until fully cooked, breaking it apart as it cooks, about 7-8 minutes.
5. Drain any excess fat from the skillet, then stir the meat and vegetable mixture together evenly.
6. In a medium bowl, combine the cream of mushroom soup, sour cream, and milk. Mix until smooth.
7. If using, stir cooked rice or egg noodles into the meat mixture now for extra heartiness.
8. Pour the soup mixture over the skillet contents and stir gently to combine everything.
9. Transfer the entire mixture into the prepared baking dish, spreading it out evenly.
10. Sprinkle the shredded cheddar cheese evenly over the top.
11. Bake uncovered for 25-30 minutes, or until the cheese is melted and bubbly and the edges are golden.
12. Remove from oven and let it rest for about 5 minutes before serving. Garnish with fresh parsley if desired.

One little trick I picked up from my grandmother is to let casseroles rest after baking—it allows the flavors to settle and makes serving a breeze. Plus, kids seem to appreciate the slightly cooler temperature!