Preheat the oven to 400˚F. Place the potatoes and onion on a rimmed baking sheet drizzle over the olive oil, salt, pepper and garlic. Using clean hands mix everything together, ensuring all of the veggies are coated with the olive oil.
Spread the onions and potatoes in a single layer on the pan. Roast in the oven 25-30 minutes, stirring once or twice during the roasting process. Potatoes should be browned and fork tender. If they are a larger fingerlings, they may need a little longer to cook.
Remove the pan from the oven and transfer to a serving platter.