In a blistering hot skillet, add olive oil and saute fennel until caramelized.
Season the sardines to taste with salt.
Lower the heat and salt the fennel to soften. In the skillet with the caramelized fennel, add the sardines and briefly saute. Add the onions, currants, and pine nuts and saffron and toss briefly.
Add tomatoes and season just the tomatoes with salt. The sauce should be slightly tight. Cook for about 10 minutes.
Bring a pot of copiously salted water to boil. Add the bucatini and cook 1 minute short of al dente. Drain the cooked pasta into the pan and toss to coat. The sauce will likely be too tight, so adjust by splashing the pan with the pasta cooking water. Pour into a serving bowl and garnish with fennel fronds and a sprinkling of the toasted breadcrumbs and red pepper flakes. Add a drizzle of your best extra-virgin olive oil and enjoy.