Onions are biennial, they will make seeds in their second year growing. That onion you planted should make a seedhead and seeds this year. If it was a hybrid variety, like most store bought onions are, the plants grown from it's seed will not be true to the parent plant.
I wouldn't bother wasting the time to grow those seeds.
However, the onion in the ground will make a nice flower that the bees will appreciate.

Thank you for that, I'll just invest in some sets for next year then