Preheat oven to 350°F.
Remove heads and tails from half the sardines. Cut in 1 inch pieces. Leave second half intact.
Wash and trim fennel. Put into pan with about 8 cups cold, salted water. Bring to a boil and cook until tender, about 5- 8 minutes. Remove with slotted spoon, drain in colander, then gently squeeze out water. Chop roughly. Reserve cooking water.
Heat 1/4 cup oil in frying pan. Sauté onions over medium heat until golden. Add chopped sardines and anchovies. As they cook, crush sardines with back of spoon to make a thick paste. Add pine nuts, raisins, saffron, salt and pepper.
In a separate pan, heat remaining oil and lightly sauté the fennel, removing to a separate bowl. Sauté the remaining whole sardines. Cook until tender, about 10 minutes, turning once gently to not break sardines.
Using fennel water, boil pasta until al dente, about 10 -= 12 minutes. Drain. Put in bowl and dress with half the sardine sauce.
Put a layer of dressed pasta in an ovenproof casserole. On top of that, put a layer of fennel, then a a layer of whole sardines with additional sauce. Continue until ingredients are used up, ending with a layer of bucatini. Cover with breadcrumbs. Sprinkle breadcrumbs over top. Cover and bake in preheated 350°F oven 15 - 20 minutes.
Pasta may be served hot or cold.