Soak the sultanas or raisins in warm water to plump up. Peel and thinly slice the onion.
Cook the fennel in plenty of boiling, salted water for 8 to 10 minutes, or until tender. Transfer with a slotted spoon to kitchen paper to dry, then roughly chop and place in a bowl, reserving the cooking water.
Heat half the oil in a pan over a medium heat and sauté the onion for about 5 minutes, or until softened. Add 500ml of the water used to cook the fennel, as well as the saffron, drained sultanas or raisins and pine nuts and simmer over a low heat for 5 minutes.
Meanwhile, sauté the anchovies with remaining oil in a small pan over low heat for few minutes until they disintegrate, then add to sauce.
Add the sardines to the pan and cook for 3 to 4 minutes, then add the fennel. Season generously with sea salt and black pepper and cook for a further 3 to 4 minutes.
Meanwhile, cook the pasta in the boiling fennel water until al dente. Drain and serve with sauce, sprinkling breadcrumbs on top.