In a large bowl, combine the ground beef, breadcrumbs, eggs, Worcestershire sauce, onion powder, garlic salt, mustard powder, salt, and pepper. Mix until just combined. Shape the mixture into four oval patties.

Heat the cooking oil in a large skillet over medium-high heat. Once hot, add the patties and cook for 2–3 minutes per side until well browned. Remove the patties from the skillet and set aside.

In the same skillet, add the butter, mushrooms, and sliced onion. Cook over medium-high heat for about 8–10 minutes, stirring occasionally, until the mushrooms release their moisture and the onions become golden brown. Stir in the minced garlic and Italian seasoning and cook for 30 seconds.

Sprinkle the flour evenly over the mushroom mixture and stir until fully absorbed and no dry spots remain. Gradually whisk in the beef broth until smooth. Increase the heat slightly and bring the gravy to a gentle boil, then reduce to low and simmer for 5–10 minutes until thickened. Season with salt and pepper to taste.

Return the Salisbury steak patties to the skillet, nestling them into the gravy. Simmer for another 5–10 minutes, spooning the gravy over the patties, until the meat is cooked through and tender.

Serve hot over creamy mashed potatoes.