Cut chicken into serving pieces. Wash and pat dry. Season with salt and pepper. In skillet heat butter, garlic, and 2/3 of oil.Brown chicken on both sides (about 10 minutes) Lower heat and continue Cooking for 25 minutes. In separate skillet heat 2-tbl oil. Saute' onions until clear. About 5 minutes. Add mushrooms and 2-tbl water; simmer 10 minutes; Add to chicken. Cover and simmer chicken 15 minutes. Pour in sherry, up heat and boil for 1 minute. Serve hot.