Step 1Fillet the rockfish, reserving the heads and bones. Cut the fillets into 1-inch pieces, salt lightly, cover with plastic wrap and refrigerate. Place the heads and bones in a large pot and add the wine and water. Bring to a simmer and cook for 10 minutes, skimming any impurities.
Step 2Trim the stalks from the fennel, reserving the trimmings, and divide the bulb into lengthwise quarters. Cut the core from the center of each quarter, then slice the fennel into ½-inch-thick crosswise slices.
Step 3Add the fennel trimmings, carrot, leek, garlic, bay leaf, fennel seeds, peppercorns, thyme, parsley and lemon slice to the pot, pressing them in with the bones but being careful not to stir, and simmer until the broth is flavorful, 35 to 45 minutes.
Step 4Pour the fish broth through a fine mesh strainer lined with cheesecloth into a bowl set over a second bowl of ice. Season with 1 teaspoon salt and whisk to cool. You should have about 4 cups of broth.
Step 5Steam the potatoes until tender, about 15 minutes. Cool, and cut each potato in half lengthwise. (The dish can be prepared up to this point a day in advance and the strained broth and potatoes refrigerated tightly covered.)
Step 6Heat the butter in a soup pot over low heat, add the chopped fennel and cook, covered, until tender, about 15 minutes. Pour the strained broth over the fennel and bring to a simmer over high heat. Add the clams and potatoes, cover, and cook until the clams open, 3 to 4 minutes. Remove the pot from the heat and add the fish pieces, pushing them gently to submerge them in the hot broth, and cover.
Step 7Set aside until the fish is cooked through, about 5 minutes. Taste the broth and add more salt if necessary. Chop together the tarragon, parsley and lemon zest.
Step 8Use a slotted spoon to transfer the fish and vegetables to heated soup plates, then ladle over some of the broth. Sprinkle with the tarragon, parsley and lemon zest mixture, and serve immediately.