Preheat oven to 375°F. Line large rimmed baking sheet with parchment paper. Combine fennel, onion, oil, and 1 teaspoon sea salt in large bowl; toss. Spread in single layer on prepared baking sheet; sprinkle with pepper. Roast until fennel and onion are very tender, about 1 hour 15 minutes. Cool slightly. Transfer fennel mixture to processor and puree until almost smooth.
Cook potatoes in large pot of boiling salted water until very tender, about 25 minutes. Drain well. Cool slightly. Place in large bowl and mash with potato masher until smooth. Mix in fennel puree.
Melt butter with rosemary and 1 teaspoon sea salt in small saucepan over medium heat. Stir butter mixture into fennel-potato puree. Mix in crème fraîche. Season to taste with more pepper and salt, if desired. DO AHEAD: Can be made 6 hours ahead. Transfer puree to large microwave-safe bowl. Cover with plastic wrap. Chill. Before serving, microwave on high until heated through, about 4 minutes.
* Sold at some supermarkets and at specialty foods stores.