Preheat oven to 350F. Cut off top 1/4 inch of each garlic head. Place garlic heads in small shallow baking dish. Drizzle oil over. Bake until golden, about 1 hour. Cool slightly. Press individual garlic cloves between thumb and finger to release garlic. Chop garlic. Melt butter in heavy large saucepan over medium heat. Add garlic, leeks and onion and saute until onion is translucent, about 8 minutes. Reduce heat to low. Add flour and cook 10 minutes, stirring occasionally. Stir in hot stock and Sherry. Simmer 20 minutes, stirring occasionally. Cool slightly. Puree soup in batches in blender or processor. (Can be prepared 1 day ahead. Cover and refrigerate.) Return soup to saucepan. Add cream and simmer until thickened, about 10 minutes. Add lemon juice to taste. Season with salt and white pepper. Ladle into bowls. Garnish with chives.