Heat the groundnut oil in a large wok or frying pan until smoking hot. Add the chillies, ginger and garlic, and stir-fry for 30-45 seconds until golden. Stir in the mixed pak choi cores and white spring onions, and cook for 2-3 minutes to soften, then add the vinegar and sugar. Mix the soy and cornflour in a small bowl, pour into the wok and cook for a few more minutes until thickened. Stir in the pak choi leaves, cucumber, oil and mushrooms. Spread on a large tray and cool completely. To make the chilli oil, heat the oils in a small pan until smoking. Leave on the heat for 30 seconds, no longer. Stir in the chillies, cool completely, then spoon into dipping bowls. Mix all the soy-vinegar sauce ingredients in a serving bowl, and set aside.