Combine ground beef, breadcrumbs, egg, diced onion, garlic powder, salt, and pepper until just mixed.
Divide the mixture into eight portions and shape into thick oval patties. Press a small indentation into the center of each patty.
Brown the patties in an oven-safe skillet over medium-high heat for 3 to 4 minutes per side. Transfer them to a plate.
Melt butter in the skillet. Cook the sliced onion until soft and lightly golden.
Stir in flour and cook for 2 minutes. Gradually whisk in beef broth, heavy cream, and Worcestershire sauce.
Return the hamburger steaks to the gravy. Cover tightly with foil and bake at 350°F for 45 minutes.
Remove the foil and bake for 15 minutes more, until bubbling and the beef reaches 160°F in the center.
Rest for 10 minutes, garnish with parsley if desired, and serve with mashed potatoes, rice, or vegetables.