Yes. Wait until the seeds are black, then gently cut off flowers and toss them in a paper bag to dry. Keep top open. Shake occasionally as the flowers dry, then remove. Slowly pour out seed into a container and store in a cool room, not the garage.
If you have no other flowering onions in your neighborhood (ornamental alliums don't count if they are not the same same species as garden onions) you stand a good chance of having your seeds grow true to the variety that was planted.
Onions take two years to get the big bulbs from seeds. Usually you grow them from "sets" which are first-year onions. To make sets, start your seed, then when large enough to handle, replant farther apart. I have no idea on the timing of this. Honestly, I grow good onion flowers, but never big onions! (Clearly I am doing something wrong, but my garden is too small to bother about tricky vegetables. I leave those to the farmers and plant kale and tomatoes and beans!)
If you have no other flowering onions in your neighborhood (ornamental alliums don't count if they are not the same same species as garden onions) you stand a good chance of having your seeds grow true to the variety that was planted.
Onions take two years to get the big bulbs from seeds. Usually you grow them from "sets" which are first-year onions. To make sets, start your seed, then when large enough to handle, replant farther apart. I have no idea on the timing of this. Honestly, I grow good onion flowers, but never big onions! (Clearly I am doing something wrong, but my garden is too small to bother about tricky vegetables. I leave those to the farmers and plant kale and tomatoes and beans!)