Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs for al dente. Reserve 1 1/4 cups cooking water, then drain. Reserve the pot.
Meanwhile, put the raisins in a small bowl and add hot tap water to just cover; set aside. Mix 2 teaspoons olive oil with the breadcrumbs and 1/2 teaspoon fennel seeds; toast in a large skillet over medium heat, stirring, until golden, about 3 minutes. Transfer to a bowl and season with salt and pepper.
Wipe out the pasta pot and heat the remaining 1/4 cup olive oil over medium-high heat. Add the onion and fennel bulb and cook, stirring occasionally, until the fennel is tender and golden, about 8 minutes. Add the pine nuts, turmeric and remaining 1 teaspoon fennel seeds and cook, stirring, until the oil is golden and the nuts and spices are toasted, 1 to 2 minutes. Stir in the raisins and soaking water and 1/2 cup of the reserved cooking water; remove from the heat.
Add the pasta, sardines and fennel fronds to the pot. Return to medium-high heat and cook, stirring, until coated, adding the remaining cooking water as needed to loosen. Season with salt and pepper. Divide among bowls, drizzle with olive oil and top with the toasted breadcrumbs.
Photograph by Ryan Dausch