Taro and Rocket Soup

Name: Taro and Rocket Soup (Yu Tou Wa Wa Cai 芋頭娃娃菜)
Cuisine: Chinese
Ingredients: 800 g taro; 3 tbsp cooking oil; 200 g rocket leaves; 1.5 litres chicken stock or water; Ground white pepper; Salt; 3 spring onions, green parts only, finely sliced