Make the Sichuan chilli oil: Heat the oil in a large, heavy-based saucepan to 85–90°C (185–195°F) and add the ginger, leek or spring onions and the garlic. They should fizz barely in the pan. If they fizz fiercely, turn down the temperature; you don’t want them to colour or burn. Once the temperature is adjusted to your liking, add the star anise and three tablespoons of the Sichuan peppercorns, along with the coriander seeds, cassia bark or cinnamon stick, black and green cardamom pods, bay leaves, cloves and fennel seeds. Turn the heat to the lowest setting and leave the oil to infuse for at least one hour, or preferably two hours. Keep an eye on the oil, stirring every now and again and making sure the aromatics are not getting too brown.
After the oil has infused, the garlic and ginger will look slightly darkened, but not browned, and a little shrivelled. Let the oil cool slightly before straining out the solid ingredients. Grind the remaining three tablespoons of Sichuan peppercorns using a pestle and mortar or a spice grinder. Mix these in a bowl with the Sichuan chilli flakes, then add to the sterilised jar in which you plan to store your oil, along with the toasted sesame seeds.
Carefully pour the warm oil over the chilli flakes. The flakes will sizzle, and the oil will turn a deep red. Once the oil has fully cooled, add the salt and soy sauce. Seal the jar with a lid and store in the fridge.
Tenderise your beef. Add to a bowl with the cornflour, rice wine and soy sauce, combine well and set aside to marinate while you prepare other elements of the dish.
Combine all the ingredients for the seasoning sauce in a shallow bowl or mug. Drain the rehydrated mushrooms, reserving the soaking liquid. Chop the spring onions into 4cm sections, then slice these lengthways to make thin matchsticks.
Cook your noodles until al dente or according to the packet and then drain. Do not cook them until soft, as they will continue to cook in the wok. Rinse these with plenty of cold water until they are completely cool (this will prevent them from sticking), then set aside.
Heat your wok over a high heat and add one tablespoon of the neutral oil. Add the marinated beef and stir continuously for two to three minutes or until the meat has browned and is crispy looking. Remove the beef from the wok and set aside, then clean the wok.
Place the other tablespoon of oil in the wok and set over a high heat. Add the garlic and quickly fry until fragrant (20 seconds), followed by the pak choi and rehydrated mushrooms, cooking for another minute or two. Keep everything moving in the pan to prevent it from burning. Add your cooked noodles, followed by the seasoning sauce and the reserved mushroom rehydration liquid. Return the cooked steak to the pan. Toss everything well and cook for one to two minutes until the sauce is bubbling and slightly reduced. Add the spring onions and toss through. Remove from the heat and serve with Sichuan chilli oil to taste.