Preheat oven to 350°F (175°C).
In a skillet, cook ground beef with chopped onion until browned. Drain excess fat.
Add mushrooms and cook until softened, about 5 minutes.
Blanch green beans in boiling water for 3–4 minutes until just tender, then drain.
Mix beef, onions, mushrooms, green beans, cream of mushroom soup, sour cream, garlic powder, salt, and pepper in a large bowl.
Transfer mixture into a greased 9x13-inch casserole dish.
Sprinkle shredded cheddar cheese evenly on top.
Bake for 25–30 minutes until the cheese is melted and bubbly.
Let cool slightly before serving.