In a hot skillet, add olive oil and saute fennel until caramelized.
Remove the heads of the sardines and pull out the backbones and entrails. Select a few sardines, for garnish. Chop the rest of the sardines for the sauce, set aside. Season the sardines for garnish, to taste with salt and pepper, and coat them with the semolina flour. In a small saucepan, heat 1/2 cup of olive oil over medium heat. When the oil begins to smoke, cook each flour-coated sardine until a light golden brown, about 1 minute on each side. Using a slotted spoon, remove the sardines from the pan and set them aside to drain on a paper towel.
In the skillet with the caramelized fennel, add the onions, currants, tomatoes, pine nuts and saffron. Season, to taste, with salt and pepper. Bring the sauce briefly to a boil and then lower to a simmer. Add the reserved sardines and cook, stirring occasionally with a wooden spoon, until the sardines have broken into pieces and are thoroughly mixed into the sauce, about 10 to15 minutes. If the sauce appears too thick at this point, add a little of the pasta cooking water.
Bring 6 quarts of water and 2 tablespoons of salt to a rolling boil. Add the bucatini and cook until tender but still al dente. Drain the cooked pasta into a large serving bowl, add 3/4 of the sauce and stir to combine. Top with the remaining sauce and the fried sardines. This pasta tastes best if allowed to sit for several minutes, soaking up the flavors of the sauce, before it is served. Keep the pasta covered during this waiting period, then garnish with reserved fennel fronds.