Place sliced potatoes, half the diced onions, and half the chopped parsley onto a double layer of foil. Drizzle with 1 Tbsp. of olive oil and season with a pinch of salt and pepper. Wrap foil around vegetables, forming a pouch. Place pouch on grill and cook for 17-20 minutes, turning occasionally, until tender. If cooking indoors, place potatoes on a baking sheet and bake for 17-20 minutes.