For the topping:
Preheat an oven to 350 degrees F.
Toss the asparagus with salt, pepper, two tablespoons olive oil, and one tablespoon lemon juice. Place them on a heavy baking sheet with sides and roast in the oven for five minutes, or until the asparagus are tender but not mushy and still bright green.
Sauté the leek in remaining olive oil over medium heat for four to five minutes, or until tender. Put the asparagus, leek, and chicken stock in a blender, working in batches if necessary, and purée the vegetables. Pour into a large bowl and add the buttermilk and remaining lemon juice. Combine well. Push the soup through a sieve or fine strainer to remove any vegetable fibers. Season with salt and pepper to taste. Chill for at least one hour. The soup may be made and refrigerated for up to one day in advance.
Whisk the cheese and cream. Fold in the truffle oil. Season with salt and pepper. Put a dollop on top of each serving of soup. Sprinkle with fresh chives.
