Boil chunks of fish until they flake apart and remove the bones. Save the water for stock. Melt bacon fat in a soup pot and add onions, celery leaves and carrots. Fry until golden.
Add 2 cups fish stock and thick-sliced or diced potatoes and slowly cook until tender, keeping just enough water in pot so they do not stick. When the potatoes are tender, add fish and milk to taste and to the consistency you prefer and simmer nearly to a boil. Add salt and pepper to taste, and 1/4 teaspoon chili powder if desired.