How to Make It
Combine the tomatoes, chopped fennel, onion, currants and olive oil in a large bowl. Season to taste with salt and pepper. Let sit for 1 hour at room temperature for flavors to meld.
With your fingers, gently break the sardines into bite-size pieces, removing any bones and scales as needed. Stir the sardines gently into the tomato sauce, taking care not to break them up too much.
Cook the spaghetti in a large pot of boiling well-salted water until al dente. Drain well and add to the bowl of sauce. Toss to combine, then taste and adjust the seasonings. Serve warm or at room temperature, garnished with the fennel fronds.