In a large bowl, gently mix ground beef, breadcrumbs, egg, diced onion, Worcestershire sauce, garlic powder, onion powder, salt, and pepper until just combined.
Divide mixture into 4 to 6 portions and shape into oval patties about ¾-inch thick. Press a shallow dimple into the center of each patty.
Heat a large skillet over medium-high heat. Sear patties 3 to 4 minutes per side until browned. Transfer to a plate.
Reduce heat to medium. Add butter to the skillet, then add sliced onions (and mushrooms if using). Cook 6 to 8 minutes until softened and lightly golden. Add garlic and cook 30 seconds.
Sprinkle flour over the onion mixture and stir for 1 minute. Slowly whisk in beef broth, scraping the bottom of the skillet as you go.
Stir in Worcestershire sauce, Dijon (if using), and black pepper. Bring to a gentle simmer.
Return patties to the skillet. Spoon gravy over the top, cover, and simmer 10 to 15 minutes, until patties are cooked through (160°F / 74°C).
While patties simmer, bake Texas toast according to package directions until crisp and golden.
To serve, place toast on plates, top with a patty, and spoon gravy and onions over everything. Garnish with parsley if desired.