My largest onions this year were the Sweet Yellow Spanish variety. I had one weighing in at 2lbs 7ozs. Probably a tiddler compared to what the competition growers produce, but big for me. I didn't do anything special, just dug the ground over well and applied some fish blood and bone once they got going. I always grow from seed as there's less chance of bolting - however - this year seemed to be an exception with more of the onions bolting than I've ever had before. I put this down to the weather. Seed is sown in a cold greenhouse any time after Christmas to the middle of January, and they seem to take forever to germinate, but I just leave them to get on with it. I usually sow seed sparingly into deep, eight inch pots and don't thin them out at all. When large enough, later in the year, I just take them to the plot, carefully knock out the pots, separate out the plants and plant them into dibbed holes. Water well, then keep watering and feeding as necessary.