After peeling and slicing the potatoes, place them in cold water while preparing the rest of the ingredients. In a large bowl mix red onion, thyme, oil, salt and pepper. Tear off 4 strips of foil about 20" long, grease the foil with a thin layer of oil. Add drained potatoes to onion mixture and divide up onto the 4 pieces of foil. Fold over edges to make flat packages, seal tightly. Roast for around 30 minutes on a heavy metal baking tray, turning a few times. Before the potatoes start to stick to the foil, remove from packages and return to tray. Roast for another 15 to 30 minutes until golden brown.