Calves Liver With Onion, Mushroom and Sage

1 lb. liver,
8 oz. onions,
6-6&1/2 oz. button mushrooms (most other mushrooms have less water content and won't work as well),
1/4 c. fresh sage,
2 large garlic clove,
3 T. butter,
1 tsp salt,
1/2 tsp. black pepper