Step 1
Cook and stir bread crumbs with 2 tablespoons olive oil in a skillet over medium heat until bread crumbs are crispy and toasted 2 to 5 minutes. Transfer breadcrumbs to a bowl to cool.
Step 2
Grind saffron threads with a mortar and pestle; pour white wine into mortar and stir to combine.
Step 3
Bring a large pot of lightly salted water to a boil. Cook bucatini in boiling water stirring occasionally until almost cooked through but firm to the bite 10 to 11 minutes. Drain reserving 1/2 cup of the pasta water.
Step 4
Heat remaining olive oil in large skillet over medium heat. Cook and stir onion and fennel with a pinch of salt in hot oil until onion is soft about 10 minutes.
Step 5
Stir raisins garlic and anchovy fillet into onion mixture; cook and stir until heated through about 1 minute.
Step 6
Pour saffron-wine into skillet; cook until wine is almost evaporated about 2 minutes. Pour 1 ladleful reserved pasta water into skillet and bring to a simmer. Stir pine nuts and red pepper flakes into sauce; simmer until flavors combine and sauce is reduced about 5 minutes.
Step 7
Stir bucatini and sardines into wine mixture; increase heat to high and cook stirring frequently and adding more reserved pasta water as necessary until sauce and pasta are heated through about 5 minutes. Remove from heat. Ladle into bowls and top with fennel fronds and toasted bread crumbs.