Bring large pot of lightly salted water to a boil.
Meanwhile, rinse and trim fennel, discarding stems but reserving fronds and bulbs. Set aside half the fronds. Chop remaining fronds; set aside. Add fennel bulbs to boiling water and cook 10 minutes. Using slotted spoon, remove fennel bulbs. Dice bulbs and set aside.
Return water to a boil and add pasta; stir to prevent sticking. Cook until tender, about 6 minutes. Drain pasta, reserving 1/2 cup cooking water.
While pasta is cooking, in large skillet, heat 1 tablespoon oil over low heat. Add onions, fennel seeds and chopped fronds and cook, stirring, until onions are tender, about 5 minutes.
In a cup, combine saffron and 1 1/2 tablespoons warm water; stir to blend and add mixture to skillet. Stir in diced fennel bulbs and cook, stirring, until fennel is very tender, about 15 minutes. Add currants and pine nuts. If mixture seems dry, add spoonfuls of reserved pasta water to moisten.
Add cooked pasta and remaining fronds to mixture. Increase heat to medium and cook, stirring, 1 minute. Drizzle with remaining 1 tablespoon oil, season with salt to taste and serve.