Heat the oil in a large stock pot over medium heat. Add the onion, shallots, fennel, and salt. Sauté until the onions are fragrant and sweating, about 10 minutes.
Stir in the garlic, chopped kipper snacks, and chili flakes, and sauté for 2 minutes. Stir in the tomato paste, coating the ingredients. Then add the tomatoes (including the juice), white wine, seafood stock, and bay leaf. Cover and bring to a simmer. Reduce heat to medium-low and simmer, still covered, for about 30 minutes.
Add the clams and mussels to the soup. Cover and simmer until the clams and mussels begin to open, about 5 minutes. Add the shrimp. Simmer on low, stirring gently, until the shrimp are just cooked through and the clams and mussels are completely open, about 5 minutes. (Discard any clams and mussels that do not open). Taste and season the soup with more salt and red pepper flakes to taste, if necessary.
Scoop into bowls, sprinkle chopped fennel fronds on top, and serve with fresh bread.