Melt 4 tablespoons of the butter in a large skillet over medium heat. Add the mushrooms, onions, garlic, lemon juice, Worcestershire sauce, and salt, and cook, stirring, until the mushrooms are tender. Stir in the parsley; pour the sauce into a small metal bowl or saucepan, cover, and keep warm. Melt the remaining 2 tablespoons of butter in the skillet. Cook the steaks over medium-high heat, turning them once, for 3 to 4 minutes on each side for medium. Serve the steaks with the mushroom sauce.