Place a large heavy-bottomed pot over medium heat and add about 3 tablespoons of olive oil. Toss in the diced onions, fennel, shallots, and minced chili peppers. Cook for 8 to 10 minutes, stirring occasionally, until softened and fragrant.

Stir in the minced garlic and fennel seeds, cooking for an additional 1 to 2 minutes until aromatic. Season lightly with salt.

Add the tomato paste, cooking alongside the mix for 2 to 3 minutes.

Pour in 1 1/2 cups stock, simmer for about 3 minutes, then add the remaining 8 cups of broth and gently simmer for 30 minutes.

Toss in the clams and mussels, cover the pot, and let cook for 4 to 6 minutes until opened.

Stir in halibut and shrimp, simmering for another 5 to 7 minutes until shrimp are pink and cooked through.

In a small pan, heat the remaining olive oil and toast bread slices until golden brown.

Ladle the stew into bowls, squeeze fresh lemon juice on top, and serve alongside toasted bread slices.