Combine the soy sauce, sugar, garlic, ginger, spring onions, sesame seeds, black pepper, sesame oil and dry sherry in a small bowl and mix well.
Cut the chicken breasts crossways to get wide 1/8 inch thick slices. Use a meat pounder to flatten the slices some more.As the meat is done put in a flat layer on the bottom of a wide shallow bowl and cover with some of the sauce. Do all the pieces like this in layers then set aside to marinate for at least one hour.
When ready to eat heat the oil in a wide non stick frying pan over a high heat. When the oil is hot put in the chicken pieces in a single layer, laying them out flat. Cook for about a minute on each side until lightly browned. Remove and put in a warmed serving dish.
When all the chicken is done put the mushrooms and chillies in the frying pan and pour over the marinade. Stir and cook for one minute then pour it over the chicken.