In a large mixing bowl, combine ground beef, breadcrumbs, egg, diced onion, 1 teaspoon of salt, and ½ teaspoon of black pepper. Mix until just combined and shape into 6-8 patties.

Heat 2 tablespoons of butter in a skillet over medium-high heat. Sear patties for 3-4 minutes on each side until golden brown. Transfer to a baking dish.

In the same skillet, heat remaining butter and sauté sliced onions for 5-7 minutes until soft.

Sprinkle flour over the onions and whisk for 1 minute. Gradually add beef broth while whisking. Cook until gravy thickens, about 5 minutes.

Stir in heavy cream, garlic powder, and remaining salt and pepper. Cook for 2-3 minutes until desired consistency is reached.

Pour gravy over the hamburger steaks in the baking dish. Preheat oven to 375°F (190°C) and bake for 20-25 minutes until bubbling and cooked through.