Create the Steak Mixture. Combine the ground beef, diced onions, egg, Worcestershire sauce, garlic powder, salt, and pepper in a mixing bowl. Mix gently until all the ingredients are well incorporated.
Prepare the Patties. Divide the mixture into equal portions and shape them into oval-shaped patties. Press the center of each patty slightly to prevent it from puffing up during cooking.
Cook the Hamburger Steaks. Add a tablespoon of oil to a large skillet over medium-high heat. Once the skillet is hot, carefully place the hamburger patties in the skillet and cook for 4-5 minutes on each side until they are browned and cooked through. Remove the patties from the skillet and set them aside on a plate.
Caramelize the Onions. In the same skillet, add the butter, sliced mushrooms, and onions. Cook the onions over medium heat until they become soft and caramelized, stirring occasionally. This process may take around 10-15 minutes.
Make the Gravy. Slowly pour in the beef broth and Worcestershire sauce, stirring constantly. Stir in sour cream. Allow the gravy to simmer until it thickens.
Finish the Gravy. Season the onion gravy with salt and pepper to taste.
Cover the Steaks with Gravy. Place the cooked hamburger steaks back into the skillet with the onion gravy. Spoon the gravy over the patties, coating them generously.
Let the Flavors Meld. Allow the hamburger steaks to simmer in the mushrom gravy for a couple of minutes to let the flavors meld together.
Serve and Enjoy! Plate the hamburger steak and enjoy!