In a heavy-bottomed soup pot or Dutch oven, heat oil over medium-low heat; add onion. Cook, stirring, until tender, about 5 minutes. Add ginger and cook, stirring, about 1 minute, until it begins to smell fragrant; add squash, sweet potatoes, potato, stock, and salt. Bring to a boil, cover, and reduce heat. Simmer 30 minutes or until vegetables are tender.
Puree soup, using a hand blender, regular blender, or food processor. Return to the pot, stir, and add sherry. Taste and adjust seasonings. To serve, heat through and ladle into bowls; garnish with yogurt.