Mix the patties – In a bowl, combine beef, onion, garlic, breadcrumbs, eggs, Worcestershire, mustard, salt, and pepper. Shape into 6 oval patties.
Brown the steaks – Heat a skillet over medium-high. Cook patties 3–4 minutes per side until browned (they will finish cooking in the gravy). Remove and set aside.
Make the gravy base – In the same skillet, melt butter. Sauté onions and mushrooms until softened, about 6–7 minutes.
Thicken the gravy – Sprinkle flour over the vegetables. Stir well, cooking for 1–2 minutes. Slowly whisk in beef broth, Worcestershire, thyme, salt, and pepper. Bring to a simmer.
Simmer the steaks – Return patties to the skillet. Cover and simmer 15–20 minutes, until steaks are cooked through and gravy is thickened.
Serve – Spoon mushroom gravy over steaks and serve hot with mashed potatoes, noodles, or rice.