Preheat oven to 350 F. Cut off top quarter inch of each garlic head. Place the garlic heads in a small shallow baking dish. Drizzle oil over. Bake until golden, about one hour. Cool slightly. (I now do the same thing with the garlic heads but I wrap the heads with oil on top of them in aluminum foil so they don't brown so quick.) Press individual garlic cloves between thumb and finger to release garlic oils. Chop the garlic.
Melt butter in heavy large saucepan over medium heat. Add garlic, leeks and onion and saute until onion is translucent, about eight minutes. Reduce heat to low. Add flour and cook ten 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 one day ahead. Cover and refrigerate.) Return the soup to the saucepan. Add cream and simmer until thickened, about ten minutes. Add lemon juice to taste. Season with salt and white pepper. Ladle into bowls. Garnish with chives