Preheat oven to 350°F (175°C).

In a large bowl, combine ground beef, chopped onion, salt, pepper, and garlic powder. Mix well and form into 6-8 oval-shaped patties.

In a large skillet over medium-high heat, brown the hamburger steaks on both sides. Remove from skillet and set aside.

In the same skillet, whisk in the flour until smooth. Gradually add the beef broth and milk, whisking constantly to prevent lumps.

Bring the sauce to a simmer, stirring occasionally until thickened. Remove from heat and stir in the sour cream and melted butter.

Arrange the hamburger steaks in a baking dish. Pour the sauce over the steaks.

Bake in the preheated oven for 20-25 minutes, or until the sauce is bubbly and the steaks are cooked through.

Serve hot over mashed potatoes, if desired.