Step 1Combine sumac, parsley, 1 tablespoon mint, 1 tablespoon lemon juice and olive oil in a shallow glass or ceramic dish. Cut each chicken breast through the middle horizontally to give 4 thin fillets. Add the chicken to the herb mixture, turn to coat. Cover and set aside to marinate for 10 minutes.
Step 2Preheat a barbecue hotplate or large grill pan to medium-high heat and spray lightly with olive oil. Grill chicken for 3 minutes each side, or until lightly charred and cooked through. Set aside.
Step 3Meanwhile, place peas in a large heatproof bowl; cover with boiling water and set aside for 3 minutes. Refresh under cold running water; drain well. Combine peas, spinach, cucumber and tomatoes in a large salad bowl. Whisk yoghurt with remaining lemon juice and mint in a small bowl to make dressing.
Step 4 To serve, thinly slice chicken and toss through the salad. Drizzle salad with yoghurt dressing and serve with a slice of wholegrain bread.