How to Make It
Fill a large pot with water, cover and bring to a boil. Add salt and the fennel and cook until tender, about 8 minutes. Remove with a slotted spoon and finely chop, discarding the core. Reserve the water for cooking the pasta.
In a large nonreactive saucepan, heat 1/2 cup of the olive oil. Add the sardines, onions and anchovies and cook over moderately high heat, stirring, until the onion begins to brown, about 5 minutes. Add the fennel, pine nuts and currants and cook for 3 minutes. Add all but 3 tablespoons of the tomato sauce, cover partially and cook over moderately low heat until thick, about 25 minutes. Season with salt and pepper; keep warm.
Meanwhile, heat the remaining 1/4 cup of olive oil in a medium nonreactive skillet. Add the reserved 3 tablespoons of tomato sauce and the bread crumbs and cook over moderately high heat, stirring, until golden and crisp, about 4 minutes. Transfer to a plate.
Return the pot of water to a boil, add the pasta and cook until al dente; drain well. Return the pasta to the pot, add the sauce and toss over low heat for 1 minute. Transfer to a warmed platter, top with 1/4 cup of the bread crumbs and serve immediately. Pass the remaining crumbs at the table.