Mix all the steak ingredients (ground beef, 1/3 can onion soup, egg, bread crumbs, salt, pepper, garlic powder, Worcestershire) until combined and set aside.

In a hot pan, sauté the sliced onion and mushrooms in 2 TBSP butter until translucent (lightly browned is fine). Reserve the cooking juices and transfer the onions and mushrooms to a bowl; set aside.

Form the meat mixture into patties and sear them in the hot pan on both sides until browned but not fully cooked through. Remove and set aside.

Add 2 TBSP butter to the pan, melt and brown slightly, then stir in the flour to create a roux. Cook briefly until the raw flour taste cooks off.

Return the seared steaks and any accumulated juices to the pan, then slowly add the beef broth while stirring to form a smooth gravy.

Stir in the remaining onion soup into the gravy and mix to combine.

Return the reserved onions and mushrooms and their juices to the pan. Cover and cook on low-medium for about 20 minutes, or until the steaks are cooked through.

Serve the Salisbury steaks over mashed potatoes and garnish with chopped chives.