Heat 2 tablespoons of the oil in a sauté pan over moderate heat. Add one clove minced garlic but do not brown.
Add the breadcrumbs and cook, stirring, until golden, about 5 minutes. Remove the crumbs from the pan and toss with the Parmesan, a little salt and black pepper. Wipe out the pan.
Bring a large pot of salted water to a boil. Add the pasta and cook until al dente.
In a large skillet, heat the remaining olive oil over medium heat. Sauté the onions, remaining garlic, and chopped fennel until lightly golden, about 10 minutes.
Add the tomato paste and anchovies and stir to dissolve. Add about half of the sardines. Lightly break up the sardines. Add the raisins and saffron along with their water. Season with salt and pepper. Cook the mixture for about 5 minutes.
Drain the pasta, reserving some of the cooking water. Add the drained pasta to the sardine mixture in the skillet along with ½ cup pasta water. Stir pasta well to coat with the sauce.
Continue to add some of the reserved pasta cooking water if the sauce is too dry. Gently stir in the remaining sardines along with the chopped fennel fronds. Cook briefly just enough to heat the added sardines. Serve pasted topped with toasted breadcrumbs.
