I grew show onions from seed for many years and my way was to save a couple of good size/shape onions in the autumn. Save them in a dry cool place and about end of Feb. replant them in large pots using general compost. Be careful not to overwater in the beginning, you will need to support the growth, they will start to send up a flower stem followed hopefully by a good flower head, at this time if you do not seem to have a good supply of bees , I would at midday shake or tap the stem to help pollination. Some people will cover the head to avoid cross pollination but I did not as saving more than one onion you hope to get the best of both worlds. Once the seed heads start to turn brown , I would then bag the head to avoid losing seed. I used to shake and shave the first fall of seed to avoid poorly set seed. I would sow in gentle heat around Christmas. I grew on the same ground for 14 years as it was a good area. The variety was Kelsea.