Preheat the oven to 400 degrees.
Place the potato and onion pieces in a sheet pan or other large baking pan. Drizzle 2 tablespoons of the oil over them and stir to coat.
Remove the skin from the chicken pieces if desired. Rub the remaining 2 tablespoons of oil over the chicken and place in the pan between the potatoes and onions. Sprinkle the rosemary on the chicken. Sprinkle the salt and pepper generously over all.
Bake for 30 minutes. Turn the potatoes and onion and continue baking until the chicken and vegetables are the desire doneness, at least 15 more minutes. Transfer to a large platter and serve immediately.