Q.Growing Big Onions
I’ve grown onions for 50 years now, and every year try to get them as big as I can, even putting them in early to try and get as much growing time as I can. Still I can never get them as big as some I saw many years ago on a allotment. This bloke had rows upon rows of at least 6-pound onions. I never saw him to ask, but I’m curious as to what he fed them, or if they were a special onion. I try ash and all sorts of feed to get 3-pound onions but never 6 pounds. Do you have any info that could help me?

Certified GKH Gardening Expert
This article will help you:
https://www.gardeningknowhow.com/edible/vegetables/onion/onion-info-tips-for-growing-big-onions.htm
I would also recommend working in phosphorous into the soil as well. That helps with root growth.