For the gravy: In the same skillet over medium heat, add the mushrooms and onion. Cook, stirring occasionally, until the onion is softened and the mushrooms are lightly browned, about 6 minutes. Add the butter and stir to melt. Add the flour and seasoned salt and stir together until no streaks of flour remain, about 1 minute. Increase the heat to medium-high and add the beef stock. Scrape the bottom of the skillet to remove any browned bits and stir until smooth. Bring to a simmer, then reduce the heat to medium low. Cook for 6 to 8 minutes, stirring occasionally, until thickened slightly. Stir in the heavy cream, ketchup, Worcestershire sauce, and dijon mustard. Allow the gravy to return to a light simmer, stirring occasionally. Add the steaks back into the skillet, spooning the gravy on top to coat. Let cook for 2 to 3 minutes to rewarm. Serve and top with parsley.
For the steaks: In a large bowl, combine the ground beef, Worcestershire sauce, ketchup, brown sugar, egg, breadcrumbs, mustard, seasoning salt, and pepper. Stir together until thoroughly combined. Divide the mixture into 6 even portions, about 4 ounces each. Firmly press into 1-inch thick, oval shaped steaks. Press lines into each of the patties with your finger to look like “steaks.” Heat a large skillet over medium heat. Add the oil and swirl to coat the pan. Place the steaks in the skillet and cook until golden, 6 to 8 minutes. Flip and cook the other side until the internal temperature reaches 160°F, 6 to 8 minutes more. Remove from the skillet and set aside.