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

Divide the mixture into four equal portions and shape each into a patty about ¾ inch thick.

Heat a skillet over medium-high heat, add olive oil, and cook the patties for 5-6 minutes on each side until browned.

In the same skillet, add sliced onions and mushrooms. Sauté for about 7-9 minutes until softened and golden brown.

Lower the heat, melt the butter in the skillet, and whisk in the flour to form a roux, cooking it for 1-2 minutes.

Gradually pour in the beef broth while whisking continuously to avoid lumps and allow the gravy to simmer until thick.

Return the browned patties and sautéed vegetables to the skillet, nestling them in the gravy, and let everything simmer for 5-7 minutes.

Serve the patties immediately with gravy, paired with mashed potatoes or egg noodles.