Mix together the soy sauce, sherry, sesame oil, garlic and ginger in a shallow dish. Add the steak and leave to marinate for about 20 minutes (or longer, if you have time).
Heat a large heavy-based frying pan or wok, add the sesame seeds and toast over a high heat, stirring, for a few minutes until golden. Tip on to a plate.
When ready to cook, heat the sunflower oil in a large frying pan or wok until hot. Add the steak with the marinade and stir fry for 3-4 minutes over a high heat until lightly browned. Remove, using a slotted spoon, on to a plate, leaving the juices in the pan.
Toss the carrots in the pan and stir fry for a few minutes, then add the mangetout and cook for a further 2 minutes.
Return the steak to the pan, add the mushrooms and toss everything together. Add the hoisin sauce and stir fry for a final minute. Sprinkle with the toasted sesame seeds and serve immediately.
