*** SOUP *** Cut off the tips of the asparagus and set aside. Roughly chop the stems into 3 cm pieces. Melt the butter in a soup pot, add the leeks, and cook over medium-high heat for 2-3 minutes, stirring as needed. Add the asparagus stems, salt and parsley. Pour in 5 cups of the stock and bring to a boil; then simmer until asparagus is just tender, about 6 minutes. Blend the soup in a blender. Return soup to pot, stir in cream, if using, and thin it with more stock if necessary. Season to taste with salt, pepper and lemon peel. In another pot, bring a few cups of water to a boil with a little salt. Cook the asparagus tips 1.5 to 2 minutes until they are done. Add them to the soup. Bring soup to a gentle simmer.