Shape meat with hands into 4 (3-inch) patties (each about 3/4 inch thick). Season both sides of patties with 1 tsp. salt, 1/2 tsp. pepper, and garlic powder.
Place a 12-inch cast-iron skillet over medium heat 1 minute or until hot. Add patties, and cook 4 minutes on each side. Transfer patties to a plate, reserving drippings in skillet.
Add oil to reserved drippings in skillet, and cook over medium heat until hot. Add onion, mushrooms, and remaining 1/2 tsp. salt and 1/2 tsp. pepper; sauté 8 minutes or until onion is slightly browned and tender.
Sprinkle flour over onion mixture. Cook, stirring constantly with a wooden spoon, 1 minute or until flour is thoroughly blended and smooth. Slowly stir in beef stock, stirring to loosen browned bits from bottom of skillet. Stir in Worcestershire sauce, and bring mixture to a light boil, stirring constantly.
Reduce heat to medium-low, add patties to skillet, and simmer 10 minutes or until patties are thoroughly cooked and gravy thickens. Place patties on individual serving plates, and top each with gravy.