In a large saucepan, heat oil over medium heat. Add onions, potato and garlic. Cook, stirring frequently, until onions are tender.

Add sweet potato and vegetable stock. Simmer, covered, for 20 minutes or until vegetables are tender. Add ginger.

In a food processor or blender, purée the soup in batches. Heat to just below boiling. Season to taste with salt and ground black pepper.