Preheat the oven to 350 F., then gather, chop, and measure all your ingredients.
Place the cut potatoes and onions in a plastic bag with the oil. Shake well to coat and pour out onto an ungreased cookie sheet along with any excess oil. Set the head of garlic on the pan, nestled in between the veggies. Make sure everything is 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 it to cool a bit. Then squeeze the garlic out of the head. Cut the onions down to smaller pieces and toss all ingredients together in a mixing bowl, then serve.
Please note that the nutrition data given here is a ballpark figure. Exact data is not possible.