Halve squash lengthwise; remove seeds. Place, cut sides up, in a shallow baking dish. Bake in a 400 degree F oven for about 1 hour or until soft. (Don't add water.) Scoop out pulp; set aside.
In a large saucepan, cook onion in butter until tender. Stir in squash and the 1/2 cup hazlenuts. Stir in broth. Cook over medium-low heat for 30 minutes.
Transfer about 1/4 of the soup to a blender container. Cover; blend until smooth. Repeat with remaining soup. Return to saucepan. Stir in cream. Season with salt, white pepper, and nutmeg. Bring just to boiling. Remove from heat. Strain mixture through a sieve, if you like. Season to taste.
To serve, ladle into individual bowls. Top with a dollop of sour cream and some of the remaining chopped hazelnuts. Makes 8 to 10 side-dish servings.
If you like, substitute acorn squash or fresh pumpkin for the butternut squash. You can use almonds, walnuts, black walnuts or pecans instead of hazelnuts.