Start by chopping the onion and mushrooms. Chop the onion, then take about 1/4 cup of it and finely chop. Add the 1/4 cup to a large bowl. Set the rest aside for the gravy.
Finely chop one cup of mushrooms and add to the large bowl. Slice about 2 and 1/2 cups mushrooms and set aside to use for the gravy. (I bought 1 pound of mushrooms for this recipe and used about 3/4 of it.)
In the large bowl with the 1/4 cup finely chopped onion and 1 cup finely chopped mushrooms, add ground beef, 1 egg, panko breadcrumbs, tomato paste, whole grain mustard, 1 teaspoon garlic (I used the jarred kind), 1 tablespoon Worcestershire sauce, 1/4 teaspoon liquid smoke, 1 and 1/4 teaspoons salt, 3/4 teaspoon pepper, and 1 teaspoon onion powder.
Use your hands to combine the meat. It's feels weird but just do it; using a spoon takes longer and you end up with tougher steaks usually. (This is true for meatloaf and meatballs as well.)
Shape the meat mixture into 6 oval shaped patties.
Heat a large skillet over medium high heat. Add 1 tablespoon oil and swirl to coat. When the oil shimmers, add as many meat patties as will comfortably fit on your pan. Don't crowd them too much or they will steam instead of sear. I cooked the meat in 2 batches.
Sear the meat for about 2 minutes, and when they are nice and browned use a spatula to flip them over. Sear the other side until browned, about another 2 minutes. It's okay if the patties are not cooked all the way in the middle.
Remove the patties (or "steaks") to a plate and repeat with the other patties.
Remove the patties and add 1 tablespoon butter to the hot skillet. Add the onions and sliced mushrooms that you prepared earlier. Saute for about 5 minutes. Add garlic and saute another minute.
In a glass measuring cup, whisk together the beef broth* and flour.
Add the broth mixture to the skillet. Add 1 teaspoon mustard and 1 tablespoon Worcestershire sauce. Stir and cook for a couple minutes until the mixture begins to simmer.
Add the steaks back to the pan. Turn the heat to a low simmer and cook for another 5 minutes or so, until the salisbury steaks are cooked through.
Sprinkle with chopped parsley and serve with my Aunt Shirley's Famous Creamy Mashed Potatoes! You cannot beat this combination!