Mix together ground beef, breadcrumbs, egg, salt, pepper, and garlic powder in a large bowl.
Divide meat mixture into four oval patties.
In a large skillet heated to medium heat, add olive oil. Place in ground beef patties and cook for about 5 minutes on each side. You are looking for a golden brown coloring on the beef. The inside doesn't have to be completely done but close.
Set aside patties on a plate after browning.
In the same skillet, add 1 T. butter to the pan along with sliced onions and mushrooms. Cook until browned and softened.
Stir flour into vegetables and cook for a couple of minutes more.
Whisk in beef broth and 1 T. Worcestershire sauce, and cook until thickened.
Taste and add more salt, pepper, Worcestershire sauce, and garlic powder until it suites your tastes.
Add beef patties back into sauce, spoon sauce over patties, and simmer for a few more minutes until they are cooked all the way through.