In a large bowl, combine the ground beef, panko breadcrumbs, Italian seasoning, egg, ketchup, Dijon mustard, Worcestershire sauce, salt, pepper and garlic.
Form into 4 oval patties about ¾ inch thick.
Heat olive oil in a large deep skillet over medium high heat. Add the steaks and cook until golden brown, about 4 to 5 minutes on each side. (Steaks do not need to be fully cooked at this time.) Remove steaks to a plate and set aside.
Add chopped onions to the skillet, season with salt and pepper, and cook until partly translucent, 3 to 4 minutes. (If the skillet needs a little more oil, add it now.)
Add mushrooms, season with salt and pepper, and cook until golden, about 3 to 5 minutes.
Turn down the heat to medium, add the butter and stir to melt. Add flour and whisk until combined with the butter.
Gradually pour in the beef stock, whisking continuously. Add the ketchup and Worcestershire sauce and season with salt and pepper as needed.
Bring to a simmer and add steaks back to the skillet. Cover and cook for another 5 minutes or until steaks are cooked through. If you notice the gravy getting too thick, add more beef stock or water.
Serve Salisbury steaks with gravy on top over mashed potatoes. Sprinkle with fresh ground black pepper and some parsley. Enjoy!
