Bring a large pot of salted water to a boil. Drop in the baby fennel and fronds and cook until tender, about 8 to 10 minutes. Drain and chop; set aside. If you can’t find baby fennel, use chopped fennel and cook with the onion.
Heat a large skillet with 1/4 cup olive oil. Add the onions and saute until soft but not browned, about 5 minutes. Add the white wine and the anchovies and break up with a wooden spoon.
Meanwhile, drop the pasta into a pot of salted water and cook until just al dente. Drain and set aside.
Add the reserved fennel, raisins, pine nuts, and a ladle of pasta cooking water to the skillet.
Cover and simmer for 10 minutes.
Add the sardines and continue to cook, uncovered, until the sardines are cooked through. Add the reserved pasta and a good drizzle of extra virgin olive oil. Stir to combine. Season with salt and pepper.
Garnish with the breadcrumbs and lemon zest.