Heat the oil in a large pan over a medium heat. Add the onion and cook for about 5 minutes. Add the garlic, chilli and spices and cook for 4 minutes. Stir frequently and add a splash of water if the pan gets dry. Stir in the mince and cook for 15 minutes, until the lamb is cooked through. Season to taste.
Meanwhile, mix the yoghurt and cucumber in a small bowl.
When you are ready to serve, toast the pittas on both sides. Split open and stuff with the spicy mince. Top with the yoghurt, tomatoes and herbs and serve.