Make the salisbury steaks. Use a box grater to grate the onion into a large mixing bowl. Add in the panko breadcrumbs, egg, and dijon and stir well. Let the mixture sit for 5 minutes. Add in the ground beef, grated garlic, worcestershire, steak seasoning, minced parsley, and 1 teaspoon of kosher salt and 1/2 teaspoon freshly ground black pepper. Use your hands to mix thoroughly until everything is well combined.
Shape the steaks. Divide the beef mixture into 6 equal portions and shape them into flat ovals. Transfer them to a parchment lined baking sheet and refrigerate for 15 minutes.
Sear the steaks. Preheat a heavy-bottomed skillet or braiser over medium heat for 5 minutes. Add the oil and, working in batches, sear the patties for 3-4 minutes per side, until browned. Transfer them to a plate when finished.
Saute the mushrooms and onions. Add the butter to the skillet and once it’s melted and foamy, add the thinly sliced onions and stir well to coat. Cook for 3-4 minutes, stirring occasionally, until the onions start to soften. Add in the mushrooms and season them with a bit of salt and pepper. Cook until the mushrooms have browned and released most of their liquid. Add the garlic and cook for another minute.
Make the gravy. Sprinkle the flour over the mushrooms and onions and cook for 2 minutes, stirring constantly, to cook out the raw flour flavor. Stir in the beef stock, madeira (if using), and dijon mustard and bring to a simmer. Stir in the worcestershire sauce and season the gravy to taste with salt and pepper. Simmer until the gravy is thickened, then nestle the beef patties in the gravy with any juices from the plate. Simmer on low heat for 10 minutes, until the patties are warmed through. Sprinkle the parsley over the top and serve right away.