Do the flowers need to be pollinated?

Just like a woman cannot get pregnant without semen from a man, a flower normally cannot go to seed without some kind of pollination. However, this doesn't necessarily need to be done by insect pollinators. Onion flowers sit close to each other, so the wind can make some pollination happen. You can also bend the flowers toward each other when the pollen is ripe and pollinate each with the other like that. Don't worry if you don't regularly get bumblebees visiting your windowsill, though; other insects like flies and such do also pollinate plants. They're just less noticeable. If you see a flying insect on your onion flowers that isn't actively eating them, it's likely an undercover pollinator.