How to Make It
In a medium saucepan, melt the butter in the olive oil. Add the leeks and cook over moderately high heat, stirring, until softened, about 3 minutes. Stir in the broccoli, garlic and stock, season with salt and white pepper and bring to a boil. Cover partially and simmer until the broccoli is tender, 20 minutes.
Meanwhile, in a small bowl, stir the sour cream with the lemon zest, lemon juice, chives and Parmesan. Season with salt and white pepper.
Transfer the soup to a blender and puree in batches until smooth. Stir in half of the lemon-chive cream. Ladle the soup into shallow bowls and serve the remaining lemon-chive cream on the side.