Heat oil or butter in a heavy-bottomed soup pot over medium heat. Add onion and cook until translucent, about 10 minutes. Add parsnips, carrots, stock, water and salt. Bring to a boil, then reduce heat to simmer. Let simmer until the vegetables are tender, about 40-50 minutes, stirring occasionally. Add water if necessary. Remove from heat. Purée with an immersion stick blender or in batches in a blender. Return to heat and add cream. Season with black pepper. Serve topped with finely chopped fresh parsley.