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Gently prick a few holes in the tofu with a toothpick (this will help the marinade to soak into it, giving better flavour), then cut into bite-size cubes.
Mix the marinade ingredients together in a bowl and toss in the tofu pieces. Set aside to marinate for 10-15 mins.
Heat a wok over high heat and add half the groundnut oil. When the oil starts to smoke, add the ginger slices and stir-fry for a few secs. Add the pak choy leaves and stir-fry for 1-2 mins. Add a small splash of water to create some steam and cook for 2 mins more. When the leaves have wilted and the stems are cooked but still a little crunchy, season with salt and transfer to a serving dish.
Rinse the wok under cold water, then reheat it and add the remaining oil. When it starts to smoke, add the tofu pieces (retaining the marinade liquid) and stir-fry for 5-10 mins. Take care not to break up the tofu as you toss it to get it browned evenly on all sides. Season with the rice wine and rice vinegar. Add the remaining marinade liquid, bring to the bubble and let the liquid reduce. Sprinkle over the chilli flakes and toss well. Spoon onto the pak choy and serve immediately with jasmine rice, if you like.