Make the Steak Patties:
In a bowl, mix ground beef, grated onion, garlic, Dijon, Worcestershire, breadcrumbs, egg, salt, and pepper until just combined. Form into 4 oval patties.
Sear the Patties:
Heat olive oil in a skillet over medium-high heat. Sear patties for 3–4 minutes per side until browned (they don’t need to be cooked through). Remove and set aside.
Start the Gravy:
In the same skillet, reduce heat to medium and melt butter with olive oil. Add mushrooms and onions; sauté for 6–8 minutes until golden and tender.
Build the Sauce:
Stir in garlic and cook 1 minute. Sprinkle in flour, stir well, and cook for 1–2 minutes. Slowly whisk in beef broth, then add Worcestershire and Dijon. Bring to a simmer and season with salt and pepper.
Simmer with Patties:
Return the patties to the skillet, cover, and simmer for 10–12 minutes until fully cooked through and the gravy is thickened.
Serve:
Plate the Salisbury steaks with plenty of mushroom gravy on top. Garnish with chopped parsley.