For the fish stock: Heat a large pot over medium heat and coat with the oil. Add the celery, fennel and onions, and season with salt and crushed red pepper. Cook until soft and very aromatic, 7 to 8 minutes. Add the garlic and cook for 1 to 2 minutes more. Add the fish bones, stir, and cook for 3 to 4 minutes. Add the white wine and reduce by half.
Add enough cold water to the pot to cover the fish bones by 2 inches, about 10 cups. Sprinkle lightly with salt. Toss in the bay leaves and thyme bundle. Bring to a boil, reduce to a simmer, and cook for 30 minutes. Strain the liquid.
For the stew: Coat a Dutch oven with olive oil. Add the celery, fennel, onions, crushed red pepper and salt to taste. Cook over medium heat until the vegetables start to soften, 3 to 4 minutes. Add the garlic and cook for another 1 to 2 minutes. Add the white wine and reduce by half. Add the cannellini beans, tomatoes, fish stock and saffron. Bring to a boil, reduce to a simmer, and cook for 12 to 15 minutes. Add salt to taste.
Meanwhile, heat a saute pan over medium-high heat and coat with olive oil. Pat the cod cheeks dry with paper towels and sprinkle with salt. Add the cod to the hot pan and cook until golden and crisp on both sides and three-quarters of the way cooked through. Remove the fish from the pan and reserve. Repeat with the shrimp and calamari.
Add the cod, shrimp, calamari and mussels to the simmering fish broth. Turn off the heat and cover, letting the fish gently cook through and the mussels open up, about 3 minutes. Discard any mussels that don't open.
To serve, evenly distribute the seafood and vegetables in bowls and top with fish broth. Drizzle with finishing oil, sprinkle with fennel pollen, and top with reserved fennel fronds. Serve with crusty grilled bread.