In a lightly greased saucepan, saute onion and garlic for about 5 minutes until tender. Stir in flour, cook over medium heat 2 minutes, stirring frequently.
Add squash and broth to saucepan; heat to boiling. Reduce heat and simmer, covered, until squash is tender, 15 to 20 minutes.
In a blender or food processor, process soup until smooth; season to taste with salt and white pepper. Serve warm, or chilled. Drizzle each bowl of soup with Cilantro Sour Cream and sprinkle with cilantro.
Cilantro Sour Cream
Mix sour cream and cilantro in small bowl, adding enough milk for desired consistency.