Peel and roughly chop the white onion. Zest and juice the lime. Put the zest to one side. Roughly chop half the mint leaves. Whizz the onion, chopped mint and lime juice in a blender with 1 tbsp oil. Or finely chop the onion and mix with the mint, lime and oil. Season with pepper.
Place the chicken breasts between 2 sheets of cling film. Take a rolling pin and bash them to a thickness of about 1 cm. Prick all over with a fork and lay in a non-metallic bowl. Massage the minty onion purée into the chicken and set aside.
Peel and dice ½ red onion and place in another non-metallic bowl. Zest the orange and set aside. Squeeze the orange juice over the onion. Stir in with a pinch of salt. Leave to macerate.
Toast 1 tsp coriander seeds in a dry frying pan for 1 min or so till they smell aromatic and pop. Tip into a mortar and grind with a pestle (or use a bowl and the end of a rolling pin or jam jar). Mix with the orange zest, 2 tbsp olive oil and some salt and pepper. Set aside.
Halve the nectarines and scoop out the stone. Finely chop them (if you want to peel them, soak them in a bowl of boiling water for 5 mins, then take off the skins). Halve the chilli and flick out the seeds. Dice the flesh finely. Mix the nectarines, lime zest and chilli together. Set the nectarine salsa to one side.
Heat a griddle pan brushed with ½ tbsp oil for 2 mins. Brush any excess marinade from the chicken breasts. Place them on the griddle. Cook for 6-8 mins on a fairly high heat, turning once, till the chicken is cooked through. Season with a little salt.
While the chicken cooks, place the couscous in a heatproof bowl. Add 175ml boiling water and cover with cling film or a plate. Leave to soak. When ready to serve, uncover and fluff up the couscous with a fork. Rake the coriander and orange zest mixture through it.
Drain the orange juice from the red onion into the couscous and stir the onion into the salsa. Serve the chicken with the couscous and salsa, scattered with the remaining mint.