Heat the oil in a large saucepan over medium-low heat. Add the leek and cook, stirring occasionally, for 5 minutes or until soft. Add the ginger and cook, stirring, for 2 minutes. Add the sweet potato, carrot and thyme. Stir to coat in the leek mixture.

Increase heat to high. Add the stock. Bring to the boil. Reduce heat to medium-low. Use the lid to partially cover, simmer for 15-20 minutes or until sweet potato is very tender. Set aside to cool slightly. Blend or process until smooth. Taste and season.

Return the soup to the saucepan, bring to the simmer. Add boiling water or more stock to adjust the consistency, if required.

Ladle the soup among serving bowls. Serve with yoghurt and bread.