The spring onions I first sowed in the ground were pathetic, the ones that did manage to get their heads above the ground either got smothered by weeds, or got yanked out with the weeds, and out of a 8ft row I managed about 4 good onions.
What I do now, is I have a plug tray (from B&Q) with 60 cells, I sow this with multipurpose compost and a pinch of seeds to each cell.
When the roots start showing out the bottom of the cells, I plant these out with an inch or 2 between each bunch
This has worked very well, and because they are in little bunches, I can pull a bunch up without worrying about disturbing the rest.