1. Preliminaries: Rinse the heads and bones, then place in a large saucepan along with the celery, onion, carrot, garlic and vegetable oil. Stir to coat the fish and vegetables lightly with oil, place over medium heat, cover and cook, stirring occasionally, for 8 minutes.

2. Simmering, straining and degreasing: Uncover and add the chile ancho, tomato, bay leaves, herbs, spices and orange zest. Pour in 10 cups water, partially cover and simmer gently over medium to medium-low heat for 20 minutes.

Strain through a fine-mesh sieve and discard the solids. Skim off the fat that rises and it is ready to use.

Traditional Variations

Sopa de Pescado: Half a recipe of this delicious broth can be turned into about 1½ quarts very good Mexican fish soup: Fry 1 small onion (sliced) in a little olive oil until lightly browned, then add 2 medium-small tomatoes (roasted or boiled, cored, peeled and chopped) or a 15-ounce can of tomatoes (drained and chopped); cook until reduced and thick. Add the broth and a big sprig epazote (or ½ teaspoon dried oregano); simmer 20 minutes. Add 1 to 1½ pounds boneless, skinless fish fillets (cubed) and simmer gently for several minutes, until the fish is tender (½pound of the fish can be replaced with 12 whole clams and ½ pound shrimp or tender-simmered squid or octopus for a sopa de mariscos). Serve with wedges of lime, plus a bowl of chopped onion and fresh coriander to sprinkle on. This is also good with ½ cup sliced, pitted green olives and/or 3 medium boiling potatoes, added with the broth.