Place the chicken breasts between two layers of clingfilm on a board and bash with a rolling pin to flatten them out. Mix together the rest of the ingredients for the chicken, then season the marinade well with pepper and rub over the chicken. Cover and leave to marinate for an hour in the fridge (ideally longer – overnight is best).
For the smashed chickpeas, toast the coriander seeds in a dry frying pan until they start to release their aroma. Remove and crush in a pestle and mortar. Heat the olive oil in a large frying pan over a moderate heat. Fry the onion for 5-6 minutes until soft. Add the garlic and coriander and continue to cook for another 2 minutes. Add the chickpeas and continue to cook until they are warm. Take a potato masher and mash so that roughly half of the chickpeas are mashed and half remain whole. Season well and squeeze in the lemon juice to taste. Keep warm.
To cook the chicken, heat a griddle pan over a high heat. Cook the chicken for 3-4 minutes on each side until cooked. Serve the chicken with the smashed chickpeas, minted yogurt and flatbreads.