Toss the lamb with the onion in a mixing bowl.
Mash the garlic with a little salt. Add the mashed garlic, one teaspoon of black pepper, cumin, cayenne and olive oil to the sliced lamb and toss well.
Cover and marinate in the refrigerator for at least 20 minutes, longer if time permits.
Heat a heavy-based frying pan or griddle until very hot.
Add the lamb to the frying pan or griddle, allowing two minutes each side. Alternately, chargrill or grill the lamb strips.
Meanwhile, warm the pitta breads. Slice along the pitta breads to form four pockets.
Fill the pitta pockets with the freshly cooked lamb, lemon juice, yoghurt, oregano and spring onions and serve at once.