Preheat oven to 350°F (175°C). Grease a baking dish.
- In a large bowl, combine ground beef, egg, bread crumbs, milk, garlic powder, salt, and pepper. Mix until just combined.
- Shape mixture into 4–6 patties.
- In a skillet, heat butter or oil over medium-high heat. Brown patties on both sides (do not cook through). Remove and set aside.
- In the same skillet, sauté sliced onions and mushrooms until softened.
- In a bowl, whisk cream of mushroom soup, sour cream, and Worcestershire sauce until smooth.
- Layer onions and mushrooms in the bottom of prepared dish. Place browned patties on top.
- Pour cream mixture evenly over patties.
- Cover with foil and bake 40–45 minutes, until patties are fully cooked and sauce is bubbly.
- Let rest 5 minutes before serving.