In a large bowl, whisk the egg, breadcrumbs, salt, pepper, garlic powder, and Worcestershire. Divide into four equal portions and shape into ½-inch-thick ovals.

Warm the oil or butter in a skillet over medium-high heat.

Cook the patties undisturbed for 3–4 minutes per side until a rich crust forms. Transfer to a plate and tent with foil.

Sauté the onions, add mushrooms, sprinkle in flour, and pour in broth. Add mustard if using.

Stir constantly to cook out raw flour taste. Slowly whisk in beef broth, scraping up browned bits.

Return the patties to the skillet, cover, and let cook on low for 8–10 minutes.

Sample the gravy for seasoning. Add more salt, pepper, or hot sauce if desired.

Sprinkle chopped parsley over patties and spoon extra gravy. Serve on mashed potatoes, rice, or noodles.