Combine the ground beef, egg, breadcrumbs, onion, garlic powder, salt, black pepper, yellow mustard, and Worcestershire Sauce in a large bowl. Mix it gently with your clean hands and shape the mixture into patties.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. sear the steak patties on both sides. Remove to a plate and cover to keep warm.
Heat the remaining olive oil and melt the butter in the skillet over medium heat. Add the mushrooms and onions.
Cook until the mushrooms are browned, and the onions are soft, about 10 minutes. Sprinkle the vegetables with the flour. Stir and cook for another 1-2 minutes.
Slowly pour the beef broth and the French onion soup into the pan, scraping the bottom to loosen any brown bits. Cook until thickened, and then add the patties back to the pan.
Spoon the gravy and onions over the patties, cover the pan, and simmer until the hamburger patties are cooked through or about 10 minutes. Sprinkle with fresh parsley or thyme and serve.