In a bowl combine the ground beef, onion, egg, ketchup, one tablespoon of mustard, one tablespoon of Worcestershire sauce, Panko breadcrumbs and salt and pepper. Knead by hand until combined. Form into 4 oval patties to give them a "steak" appearance (See Note 1).
Heat a cast iron pan or large skillet until very hot. Add 1 tablespoon of bacon fat or oil and then patties. Sear patties to a crispy brown, 3-5 minutes per side. Remove from pan and set aside on a paper towel lined plate and cover to keep warm or a casserole dish if serving later.
Add the other tablespoon of bacon fat or oil to the pan and sauté the onion until golden brown over medium heat. Add the mushrooms, remaining tablespoon of mustard and Worcestershire sauce and cook for several minutes. Mushrooms will release water so cook mixture down for several minutes. Season with salt and pepper.
Sprinkle flour over all and stir, cooking for another minute. Slowly add the beef stock, stir to mix and bring to a boil. Lower heat to simmer for several minutes, sauce will thicken. Season to taste.
Add the “steaks” back to the pan, cover and cook for 2-3 minutes to heat through. If in casserole dish to serve later: cover and cook 10 minutes in 350°F oven to heat through.
Serve over mashed potatoes and pour mushroom onion gravy over each salisbury steak. Sprinkle with parsley to garnish.