Finely slice the red onions. Slice the radish. Halve the cherry tomatoes. Dice the cucumber. Roughly chop the flat-leaf parsley. Roughly chop the mint. Juice the lemons.
- 1 Cut each chicken thigh into 1.5cm strips. Place 2 metal skewers about 5cm apart and thread a quarter of the chicken onto them, making sure it is packed quite tightly. Repeat with 6 more skewers and remaining chicken. Sprinkle with sumac and season.
- 2 Heat the oven to 220°C/425°F/gas 7 (200°C for fan ovens). Sear the chicken skewers in a hot, non-stick frying pan, then transfer to a baking tray. Roast in the oven for about 15 minutes, until golden and cooked. Remove and squeeze over a little lemon juice.
- 3 Meanwhile, shred the lettuce into 1cm strips and place in a large bowl with the onion, radishes, tomatoes, cucumber, parsley and mint. Season well. Sprinkle over the sumac and toss.
- 4 For the dressing, whisk together the remaining ingredients. Pour over the salad, toss and serve with the chicken.