I would say that your onions are flowering as they are running to seed. This can also produce what they call 'thick stem' which shortens the keeping life. Perhaps a container is not the best place to be growing onions. Best grown in the ground as they are deep feeders and their roots grow to over a foot deep

Thanks, Jimmytheone as usual for your valuable input. I have heard the roots theory before, which is why I do use deep containers and always measure the depth before putting the sets in. However, I will bear your comments in mind for next time. I have never been successful with those onions I have put in the ground though.

Perhaps try again to grow in the ground. Dig deep and loosen the soil and add some blood, fish and bone. You can sow the sets quite close and then thin them out mid season to use then, leaving the others to fatten up.

Onions flower in their second year usually. So the onion set is the first year plant.
poor weather will also 'stress' the onion and most plants when stressed produce flowers to set seed to ensure the survival of the species.

The hard bit if it is at the root end of the onion is called the basal plate and is where new roots develop from. The scale leaves of the bulb [onion] are attached to it too.

if it is in the fleshy part of the onion it will the stem of the flower stalk.

Thanks again Jimmytheone and Seaburngirl for joining in. Just to say I am vegetarian, practically vegan so would not use any animal derived fertiliser. I do use another type though. Hope anything you have both planted this Autumn will be a roaring success next Summer!