How to Make It
In a large frying pan, heat 1/4 cup of the oil over moderate heat. Add the bread crumbs and cook, stirring, until golden, about 5 minutes. Remove the crumbs from the pan and toss with the Parmesan. Wipe out the pan.
In the same pan, heat the remaining 1/4 cup oil over moderately high heat. Add the fennel and cook, stirring occasionally, until golden, 5 to 10 minutes. Add the garlic and cook, stirring, 1 minute longer. Stir in the tomatoes and bring to a simmer. Reduce the heat and simmer until thick, about 10 minutes. Add the sardines, parsley, salt, and pepper. Break up the sardines with a fork. Remove from the heat.
In a large pot of boiling, salted water, cook the spaghetti until just done, about 12 minutes. Drain. Toss the spaghetti with the sauce and half of the bread crumbs. Top with the remaining crumbs.
Variation If you're fortunate enough to find fresh sardines, substitute one pound of them, filleted, for the canned sardines. Add them to the pan along with the garlic and sauté for two to three minutes before adding the tomatoes.