Onions can be grown in pots, but you will need to grow few plants to make it worth your while. Consider growing them in small separate pots or one large container.
You don’t have to wait for all the onion bulbs to be full sized before you harvest them. You can pick and use smaller bulbs and leave the rest to grow bigger.
Onions may go to seed if they are not planted at the right time of the year, so make sure you check the label before planting so you don’t spoil your crop!