Preheat oven to 350°F (175°C).

In a large bowl, combine ground beef, chopped onion, salt, pepper, and garlic powder. Mix with your hands just until combined, being careful not to overwork the meat.

Form the mixture into 6 to 8 oval-shaped patties, about 1/2 inch thick. Make a small indent in the center of each patty with your thumb to prevent puffing during cooking.

In a large skillet over medium-high heat, brown the hamburger steaks for 3 to 4 minutes per side until a nice crust forms. The steaks do not need to be cooked through at this point.

Remove the patties from the skillet and set aside on a plate.

Reduce heat to medium. In the same skillet with the drippings, whisk in the flour and cook for 1 minute, stirring constantly, until the mixture smells nutty.

Gradually add the beef broth while whisking constantly to prevent lumps. Then slowly add the milk, continuing to whisk.

Bring the sauce to a simmer, whisking occasionally, until thickened enough to coat the back of a spoon, about 3 to 5 minutes.

Remove the sauce from heat and stir in the sour cream and melted butter until smooth.

Arrange the hamburger steaks in a 9x13-inch baking dish.

Pour the sauce evenly over the steaks, making sure each one is covered and the gravy comes about halfway up the sides.

Bake uncovered for 20 to 25 minutes, or until the sauce is bubbly and the steaks reach an internal temperature of 160°F.

Let rest for 5 minutes, then serve hot over mashed potatoes if desired.