Sauté onion, fennel, and garlic in olive oil over medium heat until fragrant and soft, about 5 minutes.

Add crushed tomatoes, saffron threads, bay leaf, thyme, and fish stock; stir well and let simmer gently for approximately 20 minutes.

Incorporate the firm white fish into the pot and cook for about 5 minutes, until the fish starts to turn opaque.

Toss in the shrimp and mussels, cooking until the mussel shells open, about 5-7 minutes.

Season with salt and black pepper before serving hot.