In a large bowl combine sumac, 1 tbsp olive oil, lemon slices, 1/2 lemon juice, then season to taste
Add chicken, toss to coat, then cover and place in fridge to marinate for 20 minutes
Preheat oven to 180°C
Cook chicken for 3 minutes each side until lightly golden
Transfer chicken to oven with marinating juices and sliced lemon and cook through. Approx 8 minutes
Remove from oven, loosely cover with foil, then rest for 2 minutes
Slice chicken and toss with chickpeas, parsley, onion and feta
Combine remaining lemon juice, olive oil and contents of baking tray and drizzle over the salad and serve.