Cook the quinoa in a pan of lightly salted boiling water following pack instructions, then drain really well.
Put the chicken breasts between two pieces of baking paper and use a rolling pin to gently tap out until less than 1cm thick. Drizzle over the oil, season well and dust with the cumin and coriander, turning to coat.
Heat a large non-stick frying pan until medium-hot then cook the chicken breasts for 5-6 minutes on each side or until golden, charred and cooked through. Move onto a plate to rest for 2-3 minutes, then slice.
Mix the onion, cucumber, tomatoes and mint with the olive oil, 1 tsp of sumac, the lemon juice and some seasoning, and mix really well, then fold in the sliced chicken, any juices and the cooked quinoa.
Pile onto plates and top with the remaining sumac and more mint leaves, if you like.