Silken Tofu with Pickled Mustard Greens

Name: Silken Tofu with Pickled Mustard Greens (Suan Cai Dou Hua 酸菜豆花)
Cuisine: Chinese
Ingredients: 30 g pickled mustard greens; Salt; 300 ml chicken or vegetable stock; 1½ tsp potato flour mixed with 1½ tbsp cold water; 2 tbsp finely sliced spring onion greens; Ground white pepper; 300 g silken tofu; 1½ tbsp cooking oil