Cut the chicken into bite-sized pieces. Rub with the salt & pepper and coat with the cornflour. Cut the asparagus diagonally into three equal parts. Slice the mushrooms 5mm thick.
Put the oil in a frying pan and add the chicken skin side down. Fry for 5 minutes on high heat without touching. Turn them over and fry for a further 2 minutes then remove to a plate.
Remove excess oil from the pan with some kitchen paper, then add 1 tbsp oil and fry the asparagus and mushrooms for 3 minutes on high heat.
Add the garlic sliced to 2mm and fry for 30 seconds. Add the butter, soy sauce, sugar and sake and fry for a further 2 minutes.
Return the chicken to the pan and fry together for 1 minute. Transfer to a serving dish and sprinkle with the sesame seeds.
The garlic and butter are the most delicious when they've cooked slightly but just before they start to burn.