How To make "Simply Chowder" Soup For a Crowd

1 large onion

chopped
1 pound sole fillet :

frozen
1/4 cup butter
7 cans cream of potato soup

(10 Oz. can)
7 cans 1% milk :

(10 Oz. can)
20 ounces clams, canned with liquid
12 ounces shrimp, canned -- small cocktail size
Saut