Onions are slow-growing and have shallow roots, and weeds are just the opposite.
Onions are slim plants, while weeds often have thick foliage that blocks out the sun onions need. All in all, onions can't give the weeds much competition. Give your onions a head start in their battle with weeds by planting them in a spot where the weeds were previously kept down. For example, put them where a wide-row of some shade crop, such as bush beans or peas, kept weeds out the previous season. There won't be as many weeds to sprout in that area, and the onions will have a better chance.