Preheat oven to 425 degrees. In a bowl, mix yoghurt, tomatoes, thyme, salt and pepper. Set yoghurt mixture aside. Rub chicken pieces with salt and pepper to taste. Arrange chicken pieces, olives and onion quarters in a single layer in a 9x12 glass baking dish or shallow casserole. Cover with yoghurt mixture. Bake in oven for 45-50 minutes, or until chicken is nicely browned. With a slotted spoon, transfer chicken pieces, olives and most of onion and tomato pieces to a platter. Puree liquid and remaining onion and tomato pieces in a food processor. Pour over chicken, or serve separately in a sauce boat. Serve with rice or cous-cous.