Preheat oven to 225F to keep the steaks warm while cooking the sauce

Mix ground beef, onion, eggs, 1 tablespoon Worcestershire sauce, parsley, 2 teaspoons salt, garlic powder, onion powder, and 1/2 teaspoon pepper together in a bowl until well combined

Divide the steak mixture into 6 portions and form into patties

Heat a large skillet over medium-high heat and cook patties, 3 at a time, until slightly browned on one side, about 5 minutes

Flip and cook until the other side is browned, about 5 minutes more, then repeat with remaining patties, pouring off any excess liquid

Place Salisbury steaks on a plate and cover with aluminum foil to maintain heat, or keep them warm in the oven

Melt butter in the same skillet over medium heat, then add mushrooms and cook until golden brown, about 5 minutes

Transfer mushrooms to a bowl, reserving butter in the skillet

Stir flour