Put the Sichuan peppercorns into a mortar and grind well. Take a sniff and revel in their strange and wonderful grapefruit smell.
Heat the rapeseed oil in large frying pan on a medium flame. Once hot, add the garlic and ginger, and stir-fry for 2 minutes. Add the mushrooms, cook for 6 minutes, then add the leek and stir-fry for a further 2 minutes, until the leek softens and unravels. Transfer the vegetables to a bowl.
Put the sesame oil, ground Sichuan peppercorns, chilli bean sauce, soy sauce and black beans into the same pan and fry for a couple of minutes, until the oil separates. Add the tofu, stir to coat, then return the vegetables to the pan. Pour over the stock, bring to the boil, then stir in the cornflour paste and heat until the sauce thickens. Take off the heat and transfer to a serving dish. Garnish with the spring onion strips and serve with plain white rice.