Pat liver dry with paper towels; set aside. In a large skillet cook onion in hot butter over medium heat until tender. Remove onion from skillet.
Add liver to skillet. Sprinkle with salt and pepper. Cook over medium heat for 3 minutes; turn. Return onion to skillet. Cook for 2 to 3 minutes more or until liver is slightly pink in center. Remove liver and onion from skillet. Stir in water, lemon juice, and Worcestershire sauce. Heat through. Pour over liver and onions.