In a large bowl, mix together the yoghurt, onion, ginger, garam masala, chilli powder and salt to taste. Add the cubes of lamb to the mixture, turning them over so that each piece is well-coated. Cover with cling film and leave to marinate in a cool place (not in the refrigerator) overnight.

Heat the oven to 190°C (375°F) Gas 5.

Remove the lamb from the marinade but do not wipe off the yoghurt coating. Thread the meat, the red and green peppers and mushrooms in turn on to 8 oiled kebab skewers.

Brush the inside of a large roasting tin with oil and place the skewers in it side-by-side. Brush the meat with a little oil, then cook in the oven for about 1 hour, until the meat is tender, turning the skewers and brushing them with more oil every 15 minutes. Transfer the skewers to a bed of plain boiled rice and serve at once, with the tasty juices from the roasting tin spooned carefully over the kebabs.