01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Heat a large skillet over medium heat. Add ground beef and cook, breaking apart with a wooden spoon, until browned and crumbly, about 6-8 minutes. Drain excess fat if needed.
03 - Add chopped onion and garlic to the skillet with the beef. Cook for 2-3 minutes until onion is softened and fragrant.
04 - Stir in sliced mushrooms and continue cooking for 5 minutes until mushrooms are tender and most of the liquid has evaporated.
05 - In a large bowl, whisk together cream of mushroom soup, sour cream, milk, Worcestershire sauce, dried thyme, salt, and pepper until smooth and well combined.
06 - Add the cooked beef and vegetable mixture to the bowl with the sauce. Stir thoroughly to coat everything evenly.
07 - Transfer the mixture to the prepared baking dish, spreading it into an even layer. Sprinkle shredded cheddar cheese evenly over the top.
08 - In a small bowl, combine breadcrumbs or crushed crackers with melted butter. Mix until evenly coated, then sprinkle over the cheese layer.
09 - Bake for 25-30 minutes until the topping is golden brown and the casserole is bubbling around the edges.
10 - Remove from oven and let stand for 5 minutes to allow the casserole to set slightly before serving.