Heat butter or olive oil in a large pot over medium heat. Add leeks (reserving 1/2 cup for garnish) and saute for about 5 minutes.
Add parsnips, apples, potatoes, 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 the pot. Stir in the heavy cream or cashew cream (see step 5). 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.
For cashew cream: grind the cashews in a mini-food processor or nut grinder (some blenders are not powerful enough to turn nuts into cream, so give them a head start). If you have a Vita-Mix, skip this step. Put the water into a blender. Add the ground cashews, lemon juice, Bragg's and nutmeg. Blend until very smooth, about 3 minutes.