Place the squash, lemongrass, ginger and stock in a stock pot. Bring to a boil, then reduce to a simmer for about 20-25 minutes or until squash is fork tender. Let the mixture cool for about 5-10 minutes. It should still be warm, but not hot. Pick out the lemongrass stalks. Puree using an immersion blender. Reheat to desired temperature when ready to serve. Spoon soup into bowls. Pour 1-2 tablespoons of coconut milk in each bowl before serving.