In a skillet over medium heat, cook the ground beef with the chopped onion and minced garlic until the beef is browned and the onion is tender. Drain any excess fat for a lighter finish.
While the beef cooks, bring a pot of salted water to a boil and cook the macaroni according to package instructions. Drain and set aside.
To the browned beef mixture, stir in the cream of mushroom soup, milk, salt, pepper, thyme, and drained mushrooms. This will create a rich and savory sauce that coats the ingredients beautifully.
In a 2-quart casserole dish, layer half the beef mixture, followed by half the cooked macaroni, and a generous sprinkling of shredded cheddar cheese. Repeat the layers, reserving a portion of cheese for the topping.
Preheat your oven to 350°F (175°C). Bake the casserole, uncovered, for 20 minutes, allowing the flavors to meld and the cheese to melt into the layers.
Remove the casserole from the oven, sprinkle the remaining cheese on top, and return it to the oven. Bake for an additional 5-7 minutes, or until the cheese is perfectly melted and golden.
Allow the casserole to rest for a few minutes before serving. Each bite delivers a cozy combination of tender meat, creamy sauce, and cheesy satisfaction.