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In a pot, cook the onion and garlic in the oil over medium heat, stirring regularly, for about 10 minutes, or until the onion softens and is shiny but does not brown. Add the fish bones and heads, tomatoes, and about 2 quarts water and simmer gently for about 30 minutes. Remove from the heat and work the mixture through a large-mesh strainer or a food mill fitted with the coarse disk into a clean pot. If you want a thinner consistency, strain it again through a fine-mesh strainer.