1. Place potatoes in a large saucepan with enough cold water to cover. Place over high heat, and bring to a boil. Reduce heat to simmer, and cook until potatoes are tender when pierced with a fork, about 20 minutes. Drain in a colander, and carefully remove skins. Pass potatoes through a ricer into a large bowl.

2. Meanwhile, coat a large saute pan with olive-oil cooking spray, and place over medium heat. Add onion, fennel, sugar, salt, and pepper, and saute until onions are tender and caramelized, about 15 minutes. If pan begins to brown before fennel and onion are cooked, add water, 1/4 cup at a time, and continue cooking. Transfer fennel mixture to the bowl of a food processor, add sour cream, and puree until mixture is smooth. Place milk in a small saucepan, and bring to a boil. Whisk milk and sour-cream mixture into potatoes until smooth and fully incorporated. Serve.