In a bowl, combine ground beef, salt, and pepper. Shape into four even patties, pressing slightly in the center to prevent doming.

Heat oil in a large skillet over medium-high heat. Add patties and cook 4 minutes per side until browned. Transfer to a plate and set aside.

In the same skillet, reduce heat to medium. Add butter and sliced onions. Sauté until golden and soft, about 6–8 minutes.

Sprinkle flour over onions and stir for 1 minute. Gradually whisk in beef broth and Worcestershire sauce. Simmer until thickened, about 3–4 minutes.

Return patties to skillet, spoon gravy over them, cover, and let cook another 5 minutes to meld flavors.

Plate the hamburger steaks, spoon extra onion gravy on top, and garnish with parsley if desired.