Peel and de-vein the shrimp; place the shells in a medium pot and reserve the de-veined shrimp in a bowl, tossing with half the ground fennel seed to coat. Use the remaining ground fennel to season the fish fillets, and set aside.
Add the stock, water, clam juice, wine, bay leaf, lemon rind and allspice berries to the pot with the shrimp shells; bring to a boil, then reduce to medium-low heat and simmer for 20 minutes.
Strain the broth through a wire strainer, discarding the shrimp shells and whole spices. Add potatoes to the broth, and simmer until fork-tender.
Heat 1 tablespoon of the oil in a 4-quart soup pot over medium heat. Add the fennel, onion, a pinch of salt and 1 tablespoon of the Piment d’ Espelette. Cook for 12 to 14 minutes, stirring a few times, until the vegetables are tender and golden. Deglaze with the tomatoes, add the garlic, and simmer until reduced by half.
Add broth to tomato and vegetable mixture, adding in the olives and sun dried tomatoes, and season to taste with salt and fresh pepper. Keep hot over medium low heat.
Heat the remaining 2 teaspoons of the oil and the butter in a sauté pan over medium-high heat. Sear the fish until lightly browned, turning and cooking until just firm - do not overcook, as they will continue to cook in the hot broth when served. Transfer to a plate.
Add the shrimp to the hot pan along with clams and/or mussels, along with a few generous ladles of broth, and cover tightly with a lid to steam open the bivalves.
Place an equal portion of fish into serving bowls. Ladle the hot broth over the fish, garnishing each serving with shrimp, clams and mussels, chopped fennel fronds and a generous pinch of Piment d’ Espelette.
