Make chicken: In a large bowl, whisk together oil, lemon juice, garlic, and seasonings. Add chicken and toss to coat. Cover and refrigerate for at least 2 hours and up to overnight.  Preheat oven to 220°C (200ºC fan) and grease a large baking tray with cooking spray. Add onion to marinade and toss to coat. Remove chicken and onion from marinade and place on prepared baking tray. Bake until chicken is golden and cooked through, 30 minutes. Let chicken rest on cutting board for 5 minutes, then thinly slice.  Meanwhile, make yoghurt sauce: In a small bowl, whisk together yoghurt, lemon juice, oil, and garlic. Season with salt and a pinch of chilli flakes. To serve as a pita, top warmed pitas with chicken, onion, lettuce, tomatoes, cucumber, and yoghurt sauce.