In a 6 quart saucepan or soup pot, heat the oil over low heat. Add the onion, celery, and garlic and cook until tender, 6 to 8 minutes. Add the white wine, tomato paste, saffron, stock, crushed red pepper, orange juice, and orange zest and bring to a simmer. Cook uncovered, for about 45 minutes, or until liquid is reduced by about one-third.
Add the fennel and tomatoes and cook until the fennel is tender, about 20 minutes. Season with salt and freshly ground black pepper, to taste.
Season the fish fillets with salt and freshly ground black pepper. Cut the fillets into 1 1/2-inch chunks. Add the fish, shrimp, and parsley to the broth and allow to cook, gently stirring only once or twice, until the fish is cooked through, about 5 minutes. Serve in shallow bowls with garlic-rubbed croutons.
Yield: 3 quarts, about 6 main-course servings