Initially you'll get green onion shoots; i.e., scallions with no bulbs.

But because this is the second year (onions are biennials) the liklihood is that it will quickly bolt and your'll get a flower head, and, later, seeds. But it will not regrow into a new bulb.

I don't think I want to get an onion out of it anyway. I would love to get some flowers for my little balcony garden though.

Do you know what type of flowers I would get from this type of onion? What do I need to do, just put the whole onion the soil. What will the onion do to the soil? I imagine it will change the PH, no?

No, it won't change the soil. Flower will probably be white. Just plant the onion with the top just below the surface of the pot.
Have you tried growing chives? They will do fine on a balcony, have purple flowers and you can snip the leaves for omelets and salads and they will go on growing. The flowers can be put in vinegar to make chive flavored vinegar for salads and after the flowers are finished you cut them off and the chives just goes on doing its thing. It may even flower again and it has lots of flowers, not just the one like an onion has.

Thanks. Do you know what the flower looks like? Is it one of the pom pom looking ones?