In a large pan that is big enough to fit the pasta, warm the olive oil over medium heat. Add the onion and fennel and sauté until the onion is tender and translucent and the fennel has almost softened, about 6 minutes.
Add the garlic, salt, pepper, fennel seeds (if using), and crushed red pepper and sauté until fragrant, about 1 minute. Add the wine and currants and cook until the liquid is almost fully reduced.
Add the anchovies and sardines, breaking them up in the pan with a wooden spoon, until heated through. Add the pine nuts and toss to combine.
Bring a large pot of water to a boil. Season the water with a handful of kosher salt. Add the pasta and cook until al dente. Reserve some of the pasta water (about 1 cup) before draining the pasta. Add the pasta to the pan with the sardine mixture and toss to combine. You can add some pasta water or a drizzle of olive oil to the pasta mixture if it looks a little dry. Garnish with reserved fennel fronds.