1. Melt butter in heavy stock pot over medium heat. Add garlic; sauté until tender.
2. Pour in broth and bring to a boil.
3. Add leeks, potatoes and squash. Return to a boil.
4. Reduce heat, cover and simmer until potatoes are tender, about 20 minutes. Cool 30 minutes.
5. Puree soup in food processor working in batches or use an immersion blender. Press soup through a strainer or sieve.
6. Stir in heavy cream. Season to taste with salt and pepper.
7. Cover and refrigerate until cold, about 4 hours.
8. Ladle soup into bowls and serve topped with chopped green onions and a spoon of sour cream.
Makes 4 servings