In a food processor or using a stick blender, puree the garlic with all the ingredients up to and including the sugar.
Place the chicken in a freezer bag, pour in the marinade, squeeze out the air. Properly seal the bag, carefully knead the meat until it is evenly coated with the marinade. Marinate in the fridge for approx. 2 hrs.
Cook the potatoes in boiling salted water for approx. 10 mins. until just soft. Drain the potatoes, mix in a bowl with the onion, thyme, oil, and salt, spread out over a perforated grill tray.
Place the grill tray on the grill with the potatoes. Wipe the marinade off the chicken, season the chicken with salt, grill with the lid down over/on a medium heat (approx. 200 °C for approx. 4 mins., with the skin side up, then turn the chicken and grill for approx. 8 mins. more until cooked.