In a heavy pot over medium-low heat, melt the butter. Add the onion, thyme, bay leaf, and a pinch of salt and sauté until the onion is tender, about 12 minutes.

Add the turnips, apples, potatoes, a pinch of salt, and one cup (eight fluid ounces/250 milliliters) water.

Cover and simmer until the vegetables and apples are tender, 10–15 minutes.

Add four more cups (32 fluid ounces/one liter) water, raise the heat to high, and bring to a boil. Reduce the heat to low and simmer, uncovered, for 20 minutes. Let cool slightly.

Working in batches, purée the soup in a blender. Return to the pot and reheat. Thin the soup with water if necessary, season with the salt and white pepper.

Ladle into bowls, and garnish with the crème fraîche, and chopped parsley. Serve right away.