This soup is absolutely delicious and makes an elegant first course for a wintertime dinner. This is my own recipe, modified from one found in Bon Appetit with the addition of milk and brown sugar, and my favorite, the cheese crumbles on top.
Melt butter in large Dutch oven or stock pot over medium-high heat. Add apples, parsnips, onion, and coriander and saute lightly until slightly softened, about 12 minutes.
Add the 3 cups stock and 1 cup milk and bring to a boil. Reduce heat to medium-low, cover, and simmer until apples and parsnips are tender--about 40 minutes.
Remove soup from heat and puree in the pot with an immersible blender, or, puree soup in a blender in small batches. Return soup to pot.
Return soup pot to medium-low heat. Add brown sugar, salt, and pepper, stir well to dissolve sugar, and reheat to eating temperature. Serve with cheese crumbles on top. Bleu cheese, roquefort, gorgonzola and fontina cheeses all taste delicious paired with this soup. Crack a bit more black pepper on each bowlful before eating.
NOTE: This soup keeps well. You can make it a day ahead and store in a covered crockpot crock in the refrigerator. Reheat later in the crockpot for serving.