In a large mixing bowl, combine the ground beef, breadcrumbs, egg, milk, ketchup, Worcestershire sauce, Dijon mustard, garlic powder, onion powder, salt, and pepper. Mix gently until just combined.

Shape the mixture into 4–6 oval patties, about ¾-inch thick.

Heat the olive oil in a large skillet over medium-high heat. Sear the patties for 4–5 minutes per side until nicely browned. Remove and set aside.

Reduce the heat to medium. Melt the butter in the same skillet and add the sliced onions. Cook for about 8 minutes until softened and lightly caramelized.

Add the mushrooms and cook another 5–6 minutes until browned. Stir in the garlic and cook for 30 seconds.

Sprinkle the flour over the vegetables and stir continuously for 1 minute.

Slowly whisk in the beef broth until smooth. Add Worcestershire sauce, Dijon mustard, thyme, salt, and pepper.

Return the Salisbury steaks to the skillet. Spoon gravy over the patties.

Cover and simmer for 15–20 minutes until the patties are fully cooked and the gravy has thickened.

Garnish with chopped parsley and serve over buttery mashed potatoes with plenty of onion mushroom gravy.