Thinly slice half of the fennel bulb and set aside. Coarsely chop the other half.

Heat 2 tablespoons of olive oil in a medium heavy pot over medium-low heat.

Add the chopped fennel, celery, leek, garlic, and bay leaf to the pot. Season with salt and pepper.

Cook, stirring occasionally, until the vegetables are soft but have not taken on any color, about 10-12 minutes.

Meanwhile, remove the shells and heads from all but 2 prawns, reserving the shells and heads.

Increase the heat to medium and add the prawn heads and shells and saffron (if using) to the pot.

Cook, stirring occasionally, until the shells are bright pink, about 4 minutes.

Add the white wine and bring to a boil. Reduce heat and simmer, stirring occasionally, until reduced by half, about 3 minutes.

Add half of the canned whole peeled tomatoes and 4 cups of water.

Bring to a strong simmer, then reduce heat and simmer, uncovered, until reduced by about one-third and flavors have melded, 60-70 minutes.

Remove from heat and puree the mixture with an immersion blender until smooth (or let cool slightly and puree in batches in a blender).

Strain the broth through a fine-mesh sieve into a large measuring glass or bowl (you should have about 3 1/2 cups).

Wipe out the pot and heat 2 tablespoons of olive oil over medium heat.

Add the fennel seeds, reserved sliced fennel, and remaining tomatoes, crushing them with your hands. Season with salt and pepper.

Cook, stirring occasionally, until the fennel is softened and seeds are toasted, about 5 minutes.

Add the strained broth and bring to a simmer.

Meanwhile, heat the remaining 2 tablespoons of olive oil in a small skillet over medium-high heat.

Season the snapper and scallops with salt and pepper.

Cook the snapper, skin side down, pressing gently with the back of a spatula to ensure contact with the pan, until the skin is brown and crisp, about 4 minutes. Turn the fish and cook until cooked through, about 1 minute more. Transfer to a plate.

Cook the scallops in the same skillet (no need to wipe out) until golden brown and just cooked through, about 3 minutes per side; transfer to plate with snapper.

Slice the squid bodies into 1/4-inch-thick rings.

Add the cockles and mussels to the pot with the broth. Cook until they just start to open.

Reduce heat to medium-low and add the squid and all prawns, making sure they're submerged in the liquid. Simmer until the cockles and mussels are opened and the prawns and squid are just cooked through, about 3 minutes (discard any cockles and mussels that do not open).

Divide the seafood stew between bowls, placing a head-on prawn on top of each.

Nestle the snapper and scallops into the broth and scatter tarragon sprigs over.

Serve with bread and butter alongside for dipping into the broth.