Prepare the Onion and Breadcrumb Mixture: Use a food processor to process the onion until grated, or finely grate it by hand. Combine with breadcrumbs.
Mix the Salisbury Steak Patties: In a large bowl, mix the onion-breadcrumb mixture with garlic, ground beef, egg, Worcestershire sauce, and Dijon mustard. Form into oval patties about ¾ inch thick.
Sear the Patties: Heat olive oil in a large skillet over medium-high heat. Brown the ground beef patties for 1 minute on each side. Remove and set aside.
Cook the Onion and Garlic: In the same skillet, sauté the chopped onion and garlic until translucent, about 2 minutes.
Cook the Mushrooms: Add sliced mushrooms and cook until golden brown, about 2-3 minutes.
Make the Gravy: Lower the heat and melt butter in the skillet. Stir in flour to form a roux, then gradually whisk in beef stock. Add water, Worcestershire sauce, salt, and pepper. Stir until smooth.
Simmer the Salisbury Steaks: Return the patties to the skillet, nestling them in the gravy. Simmer on low heat for 5 minutes, or until fully cooked and the gravy has thickened.
Serve and Garnish: Sprinkle with fresh parsley and serve hot with your favorite sides.
