Step 1Cover the raisins with hot water and set aside to soften. Place the bread crumbs in a small skillet, add just enough of the oil from the sardines to moisten and toast over medium heat until bread crumbs are golden and fragrant, about 5 minutes. Transfer to small bowl to stop the cooking.
Step 2Cook the spaghetti in a large pot of rapidly boiling, heavily salted water until it is al dente, about 12 minutes.
Step 3While the spaghetti is cooking, heat the olive oil over medium heat in a skillet large enough to hold all the spaghetti. Add the garlic clove, fennel seeds and red pepper flakes and cook, stirring occasionally until the garlic has begun to turn golden, about 5 minutes. Discard the garlic.
Step 4Remove the sardines from the oil in which they're packed, retaining the oil. Add the sardines to the skillet and cook, breaking the fish into bite-sized pieces with a spatula.
Step 5When the pasta is done, drain it, reserving one-half cup of the cooking water. Add the pasta to the skillet, along with the reserved cooking water. Drain the raisins and add them. Add the fennel fronds and parsley, increase the heat to high and cook, stirring constantly, until the water has evaporated. Taste and add some of the reserved sardine oil if a stronger flavor is desired.
Step 6Scatter the toasted bread crumbs over top and serve.