In a large bowl, combine ground beef, breadcrumbs, egg, chopped onion, garlic, Worcestershire sauce, salt, pepper, and Dijon mustard. Mix until just combined.

Shape the mixture into oval patties and press a small dip in the center.

Heat a skillet over medium to medium-high heat and add a little oil or butter. When the oil shimmers, add the patties and cook for a few minutes per side until browned. Remove to a plate.

In the same skillet, add sliced mushrooms and a bit of onion if using. Let them sit for a minute before stirring and sprinkle with salt.

Once golden, add butter and sprinkle in flour, stirring for about a minute.

Slowly pour in beef broth while stirring until smooth. Add Worcestershire sauce, pepper, and taste for salt.

Let it simmer until thickened enough to coat a spoon.

Slide the patties back into the skillet with any accumulated juices. Cover loosely and simmer gently until the patties are cooked through.