Start by gathering all ingredients and prepping them efficiently before cooking begins.
In a large mixing bowl, combine lean ground beef, egg, Worcestershire sauce, Dijon mustard, minced garlic, seasoned breadcrumbs, onion powder, salt, and pepper until well mixed. Form this mixture into four oval-shaped patties.
Heat olive oil in a large skillet over medium-high heat until shimmering hot. Carefully add each patty to the skillet and cook for about 5-6 minutes per side until browned.
In the same skillet, add sliced onions and mushrooms. Sauté until tender and moisture has evaporated, about 7-9 minutes.
Lower heat to medium, add butter to the skillet until melted. Sprinkle flour over this mixture; stir constantly for 1-2 minutes.
Gradually whisk in beef broth, ensuring no lumps remain. Incorporate Worcestershire sauce and garlic powder, then return patties to the pan. Simmer for an additional 5-7 minutes.