Pre-heat the oven to 180°C.
In a small mixing bowl, combine the yogurt and crème fraîche until smooth. Cover and chill until needed. Marinade the lamb breasts in the olive oil, red pepper flakes and seasoning for 15 minutes.
Heat a large ovenproof frying pan over a medium-high heat for 2 minutes. Add the lamb breasts and sear for 5 minutes, turning occasionally. Transfer to the oven and roast for an hour until tender. Remove and thinly slice the lamb breasts.
Arrange the yogurt mixture, onion and celery in the pittas. Top with the hot lamb and sliced mint. Arrange in a presentation basket lined with celery leaves, sprigs of mint and sprigs of mint as a garnish. Serve immediately.
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