Mix all the ingredients, except the chicken and onion/potato in a large bowl.
Cover with cling film and refrigerate for 2 hours
Remove the top shelf from the oven. Preheat the oven to 160ºC
Stand the assembled skewer upright on a baking tray and cook for 1.5 hours, check regularly after 1 hour. The chicken should be cooked through, with charred edges, and no pink remaining in the centre.