Melt the butter over medium heat in a large heavy bottomed pot or dutch oven.
Add the onion and sauté 3-4 mins. Add the garlic and continue cooking for an additional 2 mins.
Remove from the heat and puree with an immersion blender (or in batches in a blender) until very smooth. Return to low heat and stir in the nutmeg and the cheese.
Season with salt and pepper to taste and serve with additional grated cheese sprinkled over the top for garnish. The soup can be stored in the refrigerator for up to 3 days or frozen for 3 months.