We have come across many versions of gin chicken over the years, and made quite a few, mostly without writing them down because it seems easier to just slosh it all in a pan and let it do its stuff. All you do is whisk up all the marinade ingredients in a bowl, pour over the chicken and set aside for 30 mins (no more). Then grill the chicken, meanwhile simmering up the marinade with the onion in a frying pan to make a reduced sauce (about 10 mins) which you pour over the chicken and serve.
Recipe by appetite magazine at https://www.appetitemag.co.uk/gin-chicken/