Onions are biennial plants, which means the first year they make bulbs (which you typically eat). If you don’t harvest onions the first year, they will come back the second year and go to seed (flower).
Last year we missed quite a few onions, and I transplanted them into the back of my perennial herb and flower beds. They are now as tall as I am, and just starting to flower!
Then what do You do with the seeds?