Mix the ingredients for the sauce in a small bowl and set aside.
Heat the oil in a wok or a large frying pan until very hot. Dredge the steak slices in the seasoned cornstarch then carefully place them into the hot oil.
Fry for 5-6 minutes, moving around the wok a few times, until the steak is well seared, then remove from the pan and place in a bowl to rest.
Discard all but 2 tbsp of the oil and fry the mushrooms, bell pepper, snow peas, garlic and ginger for 2-3 minutes on a medium heat.
Turn the heat up high and pour in the sauce. Bring to the boil and then simmer it for a 2-3 minutes until it reduces slightly. Add in the cooked steak and bean sprouts and cook for a further 2-3 minutes until the mung bean sprouts are heated through, but still crunchy.
Serve over rice or noodles. Garnish with sliced scallions.