1. Melt margarine in a large frying pan over low heat.
2. Add the chopped onion, carrot, and celery and sauté about 10 minutes, stirring occasionally.
3. Add the chicken and beef stocks, squash, and potato.
4. Raise the heat and bring to a simmer. Cover pan. Simmer for about 30 minutes until the mixture is very tender.
5. Transfer the mixture to a food processor and puree until smooth. Stir in topping, mustard, and nutmeg (if using), and then salt and pepper, to taste. Refrigerate.
6. Adjust seasoning to taste. Garnish with carrot and chives just before serving. Serve cold.