Simmer haddock in 2 cups of water for 15 minutes. Drain off and reserve the broth. Remove the skin and bones from the fish. Saute the diced salt pork in a large pot until crisp. Remove salt pork and saute the onions in the pork fat until golden brown. Add fish, celery, potatoes and bay leaf. Measure reserved fish broth, plus enough boiling water, to make 3 cups liquid. Add to pot and simmer 30 minutes. Add milk and butter and simmer for an additional 5 minutes, or until well heated. Season with salt and pepper. Makes 8 servings.