Step 1Heat the olive oil in a large, heavy-bottomed stockpot over medium heat. Add the onions and leeks and sweat, stirring often and making sure the onions and leeks do not color, for 10 minutes. Add the garlic and continue to sweat, stirring often and lowering the heat if necessary, until onions and leeks have "melted" into the olive oil, about 15 minutes.
Step 2Add the fish, increase the heat to high and cook, stirring often and vigorously without concern about crushing, mashing or otherwise bruising the fish parts, until the fish falls apart and begins to dissolve into the soup, about 10 minutes.
Step 3Add the tomatoes, orange peel, celery, thyme, bay leaves, red pepper flakes and fennel stems or pastis. Lower heat a little to prevent burning, and cook, stirring frequently, for 10 minutes.
Step 4Pour the boiling water over all, lower the heat, and cook at a simmer for 25 minutes.
Step 5Pour some of the mixture through a food mill or chinois, working with small quantities of fish scraps and vegetables, wetting them down with the bouillon to ease the flow. Be sure to press down on the fish scraps with a pestle or wooden spoon so their juices seep out. Do not give up early. Some of the tastiest and richest juices will be the last to be extracted. Use the hot bouillon judiciously, periodically wetting each batch of fish scraps to help you press out the juices. When you are sure there is no more juice to be had, discard the fish scraps and begin anew with another batch of fish scraps until all the mixture has gone through the mill.
Step 6Season to taste with salt and freshly ground pepper. Store in the refrigerator for up to two days or freeze until ready to use.
Variation: If the supply of rockfish is limited, the juices of steamed mussels will add richness to the bouillabaisse broth. Place 24 to 30 scrubbed and debearded mussels and 1 cup water in a pot over medium heat, cover, and steam the mussels, shaking the pot frequently, until the mussels open, 7 to 9 minutes. Remove the mussels with a slotted spoon, set aside and, if not preparing the bouillabaisse until hours or days later, refrigerate. Pour the mussel broth into the fish broth.