onions:
In a bowl combine vinegar, sugar, cinnamon and nutmeg. Stir in onions. Cover and refrigerate at least 1 hour.
soup:
In a large pot sauté celery and onion in oil over medium-high heat. Add sweet potatoes and broth. Bring to boiling. Reduce heat; simmer 20 minutes or until potatoes are tender. Season with salt, pepper and allspice. Stir in cream.
Working in batches, ladle soup into a blender or food processor. Blend until smooth and transfer to a clean pot. Or, use an immersion blender to blend soup in pot until smooth. Adjust seasonings to taste.
To serve, ladle soup into soup bowls and top with marinated onions. Makes 4 to 6 servings