I do fork tender liver and onions like this:
Slice liver, I like it about 1/2" thick. Trim all the membrane off at the edges. Dredge in seasoned flour, brown in a mixture of oil and butter turning once or twice. Add sliced onions and a cup or so of water, or beef broth and cover with a lid. Simmer for 20-30 minutes, or until your desired degree of tenderness is reached. The liver needs to be braised to get it tender. This method works every time.