"Fish and seafood are so important in Venice," says Francesco, "that when making fish stews and risottos with fish, we prefer a fish stock. For shrimp, lobster, and crabs a shellfish stocks is better." None are complicated to prepare.
Place the fish head and bones in a large stockpot. Add the cold water, bring to a simmer, and cook for 5 minutes, skimming any foam from the surface.
Add the carrots, celery, onion, and tomato.
Continue to simmer for 20 minutes.
Strain the stock, season lightly with salt, and allow to cool to room temperature.
Refrigerate or freeze until ready to use.