In a large bowl, combine the ground beef, breadcrumbs, chopped onions, egg, Worcestershire sauce, salt, and pepper. Mix until just combined.

Shape the mixture into four equal patties, about 1 inch thick. Set aside.

In a large skillet, heat a drizzle of oil over medium-high heat. Add the patties and cook for about 4-5 minutes on each side until browned.

Remove the patties from the skillet and add the butter and sliced mushrooms to the same pan. Sauté until the mushrooms are tender and golden brown.

Sprinkle the flour over the mushrooms and stir to combine. Gradually whisk in the beef broth, scraping up any brown bits from the bottom of the pan. Continue whisking until the mixture thickens.

Return the patties to the skillet and spoon the gravy over them. Let everything simmer together for about 5 minutes.