Clean and cut the fish in 3/4-inch slices, reserving the heads and bones. Sprinkle the slices with lemon juice and dust with flour, salt and pepper. Fry in olive oil until tender. Make a fish stock by cooking the heads and bones in the water with the onion, parsley, celery and bay leaf. Simmer for 1/2 hour. Strain the stock and remove any edible fish particles from the heads. Add these to the stock with the fried fish and ~name. Cook slowly until the ~name is tender. Correct the seasoning. A little aji chombo may be added for a "hotter" soup.