Heat butter in a large pot over medium heat. Add leeks (reserving 1/2 cup for garnish). Cook, stirring, 5 minutes.
Add parsnips, apples, potato, broth, and 4 cups water. Bring to a boil; reduce heat and simmer, partially covered, until vegetables are tender, 20 to 25 minutes.
Working in batches, puree soup in a blender until smooth. Return it to pot; stir in cream. Season with salt and pepper. Serve with Leek Garnish.
To make leek garnish, in a large skillet, heat 1 tablespoon butter over medium-high. Add reserved 1/2 cup leeks; cook, stirring, until golden brown, about 3 minutes.