01 -
In a medium bowl, combine ground beef, breadcrumbs, egg, finely chopped onion, Worcestershire sauce, Dijon mustard, salt, and pepper. Mix thoroughly until well combined.
02 -
Divide the mixture into 6 equal portions and shape each into a patty approximately 3 inches in diameter. Set aside on a parchment-lined board.
03 -
Heat vegetable oil in a large skillet over medium heat. Add patties and cook for about 5 minutes per side until browned and cooked through. Remove patties and set aside.
04 -
In the same skillet, melt butter and sauté sliced onions until soft and golden, about 5 minutes. Add mushrooms and cook until tender, approximately 3 minutes.
05 -
Sprinkle flour over the onions and mushrooms; stir continuously for 1 minute to cook the flour.
06 -
Whisk together beef stock, tomato paste, Worcestershire sauce, pressed garlic, Dijon mustard, thyme, salt, and pepper in a small bowl. Gradually pour into the skillet, stirring constantly to prevent lumps. Bring to a gentle simmer.
07 -
Return beef patties to the skillet. Cover and reduce heat to medium-low. Simmer for 15 minutes, flipping patties halfway through cooking to ensure even heating.