Snap the lower end of the asparagus where they break easily when bent. Rinse the ends well and roughly chop them into 1" pieces. Combine the rest of the stock ingredients in a stock pot, bring to a boil, simmer 20 to 25 minutes, and then strain. Cut off the tips of the asparagus and set aside. Roughly chop the stems into 1" pieces. Melt the butter in a soup pot, add the leeks, and cook over medium-high heat for 2 to 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 1½ to 2 minutes until they are done. Add them to the soup. Bring soup to a gentle simmer.