Melt butter in a soup pot set over medium heat. Add onion and cook until tender. Add next 7 ingredients, reserving 4 thin slices of the ginger to garnish the top of soup, and bring to a simmer. Simmer 20-30 minutes, or until squash and apples are very tender. Puree mixture in a food processor or blender. Return to the pot. Bring back to a simmer. Season with salt and pepper. Pour into heated soup bowls. Garnish each with a spoon of yogurt, a herb sprig, and a slice of ginger.