Stir in lamb. Cover and leave to marinate for at least 4 hours.
Heat oil in a deep, round-bottomed frying pan, (karahi) or wok. Add lamb, leaving excess marinade in bowl. Cook lamb for 7 minutes, stirring frequently.
Stir in peppers, cook for 1 minute, then stir in the mushrooms and tomato purée. Cook for a further 4 minutes, stirring frequently.