In the past I have harvested seeds for cilantro which worked so well I have decided to try more this year. Now my gigantic onions are producing seed stalks and umbels are beginning to open seems like a good time to try harvesting my own onion seeds.
To save these seeds it is pretty simple, when the umbels have dried out and begin to open. At this time carefully cut them off (making sure that seeds do not fall) and place in a them in a cool dry location for 2-3 weeks. I prefer to put my harvest seeds in the refrigerator in a paper bag. It is a good idea for onions to cross pollinate so I will plan on buying some other onion seeds to mix with the ones I am harvesting to allow for cross pollination.
Onions take 100-120 days from seed to maturity, if you start from sets which you grow from the previous year can shave 3-4 weeks off that time. I could make my own onion sets by planting 30-40 seeds in a square foot area during late summer. After two months rake over the tops and let them turn brown and dry out then store them in a dry place until next spring.
My PC grow box will be empty 10 weeks before my last frost date so planning on starting my onion seeds in there moving them outside 4 weeks before the last frost.
If you want to learn more about saving seeds for onions or other vegetables be sure to check out International Seed Saving Institute which has great advice about saving seeds from dozens of different plants.