For the salad, cut the cucumber lengthways into short strips. Toss with the beansprouts and watercress and set aside in the fridge until needed.
Whisk together the soy sauce, sesame oil, sugar and salt.
Cut along one side of each squid pouch and open it out flat. Score the inner side in a diamond pattern with the tip of a small, sharp knife and then cut into 5cm/2inch squares. Separate the tentacles if large.
Heat a small, heavy-based frying pan over a high heat. Add the black peppercorns and Sichuan peppercorns and dry-roast them for a few seconds, shaking the pan now and then, until they darken slightly and become aromatic. Tip into a mortar and crush coarsely with the pestle, then stir in the sea salt flakes.
Heat a wok over a high heat until smoking. Add half the oil and half the squid and stir-fry it for 2 minutes, until lightly coloured. Tip on to the plate, then cook the remaining squid in the same way.
Return the first batch of squid to the wok and add 1 tsp of the salt and pepper mixture (the rest can be used in other stir fries).
Toss together for about 10 seconds, then add the red chilli and spring onions and toss together very briefly. Divide the squid between 4 serving plates. Toss the salad with the dressing and pile alongside the squid, then serve immediately.
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