Preheat oven to 350°F (175°C) and grease a 9x13 inch casserole dish with butter.

In a large skillet, cook ground beef over medium heat until browned, breaking it up with a spoon. Drain excess fat.

Add chopped onion and sliced mushrooms to the skillet. Cook until vegetables are softened, about 5 minutes.

In a bowl, mix bread crumbs, milk, salt, and black pepper until combined.

Combine the bread crumb mixture with the cooked beef and vegetable mixture. Stir well.

Spread the mixture evenly into the prepared casserole dish. Sprinkle shredded cheddar cheese on top.

Bake in the preheated oven for 40-45 minutes until bubbly and cheese is melted and golden.

Let the bake cool for 5 minutes before serving.