Clean the sardines: remove heads, guts, and backbones, leaving fillets attached at the tail. Rinse and pat dry.
Bring a large pot of salted water to a boil. Add the wild fennel (if using fresh) and cook for 2 minutes. Remove fennel with a slotted spoon, chop roughly, and set aside. Reserve the cooking water.
In the same water, cook the pasta until al dente. Reserve 1 cup of pasta water before draining.
While pasta cooks, heat olive oil in a large skillet over medium heat. Add onion and cook until soft, about 5 minutes.
Add pine nuts and drained raisins; cook for 2 minutes. Stir in tomato paste and cook for 1 minute.
Add the sardine fillets and cook for 2 minutes per side, breaking them up slightly with a spatula.
Pour in the white wine and let it reduce by half, about 2 minutes. Add the saffron water and chopped fennel (or fennel seeds). Season with salt and pepper.
Add the drained pasta to the skillet along with a splash of pasta water. Toss well to combine, adding more pasta water if needed for a saucy consistency.
Serve immediately, topped with toasted breadcrumbs.
Tips
If fresh sardines are unavailable, high-quality canned sardines in olive oil can be substituted; drain and use 2 cans.
Wild fennel is traditional; if unavailable, use fennel fronds or a pinch of fennel seeds.
Toasted breadcrumbs add a crunchy texture; you can also use pangrattato.