Cut the aubergines into batons about 2cm thick and 7cm long. Sprinkle with salt, mix well and set aside for at least 30 minutes.
Rinse the aubergines, drain well and pat dry with kitchen paper. Heat the deep-frying oil to around 200°C (hot enough to sizzle vigorously around a test piece of aubergine). Add the aubergines, in two or three batches, and deep-fry for about three minutes, until tender and a little golden. Drain well on kitchen paper and set aside.
Carefully pour off all but 3 tbsp oil from the wok and return to a medium flame. Add the chilli bean paste and stir-fry until the oil is red and fragrant: take care not to burn the paste (move the wok away from the burner if you think it might be overheating). Add the garlic and ginger and stir-fry until they smell delicious.
Tip in the stock or water, sugar and soy sauce. Bring to the boil, then add the aubergines, nudging them gently into the sauce so the pieces do not break apart. Simmer for a minute or so to allow the aubergines to absorb the flavours.
Give the potato starch mixture a stir and add it gradually, in about three stages, adding just enough to thicken the sauce to a luxurious gravy (you probably won’t need it all). Tip in the vinegar and all but one tablespoon of the spring onion greens, then stir for a few seconds to fuse the flavours.
Turn out on to a serving dish, scatter over the remaining spring onion greens and serve.