Heat wine in a small saucepan (or in the microwave) until it begins steaming (but not boiling). Remove from heat.
Add currants, red pepper flakes, and saffron (if using) to the wine. Set aside.
Add 1 TB olive oil to a large pan over medium-high heat until shimmering.
Add fennel seeds and breadcrumbs, and stir until breadcrumbs are toasted and the spices are fragrant (about 2-3 minutes). Remove breadcrumbs from pan and set aside.
Bring a large pot of water to a boil. Once the water reaches a rolling boil, add the bucatini.
Boil until al dente (about 6 minutes). Reserve 1 cup of pasta water, and remove pasta from the heat.
Meanwhile, heat remaining olive oil over medium heat in the large pan.
Add the diced onion, fennel bulb (not fronds), and salt. Cook about 8 minutes, until the onion and fennel have softened.
Add garlic and anchovies, and stir until the anchovies dissolve (about 2-3 minutes).
Increase the heat to medium-high. Add the reserved wine mixture to the pan, and simmer until most (but not all) of the wine has boiled off. (Stir often.)
Add the pine nuts and sardines. Stir gently for about 2 minutes, or until the fish is heated through. Remove from heat.