You may plant in clumps if you want more produce per square foot even though they will be a little smaller. Set seedlings out 1-2″ deep and 6-8″ apart in shallow trenches, 1-2″ between rows.
Onions are triggered to form bulbs in response to day length. Day length differs depending on latitude, so different onion varieties were developed to have different day-length needs. In the north, the earlier onions are set out, the more chance they have to make top growth while the days are lengthening. High fertility and steady water is crucial for large onions. Side dressing is recommended. After summer solstice they begin bulbing.
Harvest sweet onions when the bulb reaches a decent size and storage onions when the leaves are 2/3rds brown.