In a large bowl, combine ground beef, breadcrumbs, egg, milk, Worcestershire sauce, salt, pepper, and garlic powder until just incorporated.
Form the mixture into 4 oval patties, approximately 3/4-inch thick.
Heat vegetable oil in a large skillet over medium-high heat; sear patties for 3-4 minutes per side until browned, then remove and set aside.
Reduce heat to medium and melt butter in the same skillet; add onions and mushrooms and sauté for 5-7 minutes until caramelized.
Sprinkle flour over the vegetables and stir constantly for 1 minute.
Gradually whisk in the beef broth and bring to a gentle simmer, stirring until the gravy begins to thicken.
Return the beef patties to the skillet, spooning gravy over them.
Cover and simmer for 10 minutes or until the internal temperature of the beef reaches 160°F (71°C).