Heat oven to 450° F. Place the onion in a large roasting roasting pan. Pat the chicken dry with paper towels. Tuck the wings under the chicken and place it on top of the onion. Rub the chicken with 1 tablespoon of the oil and sprinkle with 1/2 teaspoon each salt and pepper.
Scatter the potatoes around the chicken, drizzle with the remaining tablespoon of oil, and sprinkle with ½ teaspoon each salt and pepper.
Roast, tossing the potatoes once, until a thermometer inserted into a thigh registers 165°F, 50 to 60 minutes. Let the chicken rest at least 15 minutes before carving. Serve with the potatoes and onions.