Wash and dry the monkfish tail. Using a sharp knife, cut through the bone to produce large chunks.
Heat half the oil in a saucepan over a low heat, add the onion, garlic and tomatoes and cook, stirring occasionally, for 10 minutes. Add the monkfish, saffron threads and stock and bring to the boil. Reduce the heat and simmer gently for 5 minutes, then remove the fish with a slotted spoon and set aside. Simmer the broth gently for a further 25 minutes.
Meanwhile, heat the remaining oil in a flameproof casserole over a medium heat. Add the pasta and cook, stirring constantly, for 5 minutes until golden.
Gradually stir in the tomato broth and simmer gently, stirring, until the pasta is cooked. Add the mussels, stir well and then add the monkfish. Cook for a further 5–6 minutes until the mussels have opened and the monkfish is cooked through. Season with salt and pepper and serve with garlic mayonnaise.