Trim fennel tops, reserving trimmings for stock and fronds for serving, and chop fennel finely. Heat oil in a large saucepan or casserole over medium heat. Add fennel, onion, celery, garlic, leeks and fennel seeds, and cook, stirring occasionally, for 10 minutes or until vegetables soften.
Meanwhile, combine saffron and 125ml of fish stock in a small bowl and set aside for 10 minutes to steep.
Peel tomatoes, remove seeds and discard, then chop finely. Add tomato paste to vegetables in pan and cook, stirring, for 1 minute. Add the saffron mixture, tomatoes, bay leaves, thyme, orange peel and the remaining stock to the pan and simmer for 30 minutes. Season generously with salt. Stir in cayenne and Pernod.
Add the fish to the pan, then prawns and scampi, making sure the seafood is submerged in the liquid. Cook for 5 minutes or until seafood is just cooked. 5 Meanwhile, place mussels, clams and 2 tablespoons water in a frying pan over high heat. Cover with a lid and cook, shaking the pan occasionally, for 3 minutes or until the shells open.
Divide bouillabaisse among bowls. Top with clams and mussels and scatter with reserved fennel fronds. Serve with toasts and rouille