Order the fish skinned, but head and tail left on. Cut off head and tail and remove fish from backbone. (You can also have your fishmonger fillet the fish for you, ask him to give you backbone, head and tail extra). Cut fish in 2-inch pieces and set aside. Put head, tail and backbone, broken in pieces, in deep kettle. Add 4 cups cold water, bay leave, thyme, onion and celery and simmer slowly for 10 minutes. Drain and reduce to 2 cups stock. Reserve. In a skillet over medium heat, render bacon, add onion and cook 5 minutes. Add potatoes and 2 cups boiling water and cook 5 minutes. Add fish stock. Add fish, cover and simmer 10 minutes. Add half-and-half, salt, pepper and butter.