broccoli and sweet potato soup

Serves: 4

Ingredients:
1 brown onion, finely chopped
500g broccoli, stems finely chopped, florets coarsely chopped
500g sweet potato, peeled, finely chopped
4 cups  vegetable stock
1/2 cup greek-style yoghurt

Method:
heat a medium saucepan over medium heat. add the onion, broccoli stems and sweet potato and cook, stirring, for 5 mins or until the onion softens.
add the stock and 2 cups (500ml) water. bring to the boil. reduce heat to low. add broccoli florets. cook for 15 mins or until sweet potato is tender. set aside to cool slightly.
use a stick blender to carefully blend the soup until smooth. divide among serving bowls. place yoghurt in a small bowl and stir in 2 tablespoons water. add to the soup and gently swirl. season.