Heat the oil in a large wok, add the mushrooms and cook for 2 minutes. Add the leek and cook for 2 minutes more until softened. Stir in the garlic and ginger, followed by the black bean and chilli sauces and the Sichuan pepper.
Mix together the cornflour, stock, soy sauce, vinegar and sugar and add to the wok. Carefully stir in the tofu. Leave to simmer for 2–3 minutes until the sauce has thickened. Sprinkle over the spring onions and serve with plain boiled rice.