Peel and roughly chop the onion. Pop it into a blender. Zest the lime into a mixing bowl and set aside, then juice the lime into the blender. Chop half the mint leaves, add them to the blender with 1 tbsp oil and whizz till smooth. No blender? Finely chop the onion and mint, then stir together with the lime juice and oil. Season with a crack of pepper.
Place the chicken breasts between 2 sheets of greaseproof paper. Bash with a rolling pin, meat mallet or pestle to a thickness of about 1cm. Prick the chicken all over with a fork and lay in a nonmetallic bowl. Massage the minty purée into the chicken and set aside.
Peel and dice the red onion and place in another non-metallic bowl. Zest the orange into a small bowl and set aside. Squeeze the orange juice over the onion. Stir in a pinch of salt. Scrunch together with your hands, then set aside to lightly pickle.
Stir the orange zest, 1 tsp ground coriander, 2 tbsp olive oil and some salt and pepper together to make a dressing. Set aside.
Halve the nectarines and scoop out the stones. Finely chop the nectarines. Halve the chilli and flick out the seeds and white pith for less heat. Dice the chilli. Mix the nectarines and chilli with the lime zest. Set the nectarine salsa to one side.
Heat a griddle pan brushed with ½ tbsp oil for 2 mins. Brush any excess marinade off the chicken breasts. Place them on the griddle. Cook for 8 mins on a fairly high heat, turning once, till the chicken is cooked through, with no pink remaining in the middle. Season with a little salt.
While the chicken cooks, place the couscous in a heatproof bowl. Add 175ml boiling water and cover with a plate. Leave to soak. When ready to serve, uncover and fluff up the couscous with a fork. Fork 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 nectarine salsa. Serve the chicken with the couscous and salsa, scattered with the remaining mint leaves.