1. Have liver cut in uniformly thick slices (1/2 inch or a little less); trim off membrane and remove large tubes; wipe with a clean damp cloth.
2. Saute the mushrooms in the butter or margarine 5 minutes, add salt and pepper then push them to 1 side in the frying pan.
3. Saute the liver quickly, no longer than 2 1/2 minutes per side in the hot butter or margarine. Remove the liver to a heated serving dish and keep it hot.
4. Add the catsup, lemon juice and wine or vinegar and water. Heat 2 or 3 minutes. Spoon this sauce over the liver.