After searching the Internet and joining a gardening message board I found out that those onions are not Walla Walla onions but “Egyptian Walking Onions”
They are a perennial and seed themselves. When they get big enough they get top-heavy, bend down to the ground and the little bulbils root in the ground. I think they also multiply underground. They are very tasty. I broke off 2 bulbils and planted one in the ground right away and put the other one in a glass of water on my dining table which started some roots in the meantime. The one in the garden seems to be growing too.
When I wanted to buy Walla Walla onions last year they must have given me the wrong seedlings. I am glad they did because it is fun to see the Waling onions grow.
I have been using some of the bulbils in my salad. Yummy.
Bye for now.