Place potato slices and onion rings on a large sheet of aluminum foil (you might need to divide the mixture between more than one piece of foil). Drizzle with oil.
Place another sheet of foil over potatoes and seal along edges.
Place on grill or in oven for 20-25 minutes or until potatoes are fork tender.
In a small bowl, mix the butter, sugar, salt and cinnamon. Stir in pecans.