Place the cut potatoes in a plastic bag with the oil. Shake well to coat and pour out onto an ungreased cookie sheet, but don't pour out all the oil yet.
Put the onions in the same bag and toss that to coat with oil as well. Toss the onions and potatoes separately because you will be chopping the onions later.
Set the head of garlic on the pan for roasting as well.
Pour the onions out onto the same cookie sheet along with any remaining oil.
Spread out so the vegetables are in a single layer.
Roast the vegetables for approximately 40 minutes, or until they are soft and cooked through and the potatoes are slightly golden edges. The garlic should have one or two cloves popping out to let you know it's done.
Allow to cool a bit. Then squeeze the garlic out of the head.
Cut the onions down to small pieces and toss all ingredients together in a mixing bowl.
Serve as a side dish.
Please note that the nutrition data given here is a ballpark figure. Exact data is not possible.