Your onion is flowering and will eventually produce seed.

You could save the seed and try planting it in the Fall but since you don't know what kind of onion it is (short-day, day-neutral, or long day, hybrid or open pollinated), you may get unexpected results from growing the seed. You could also plant the seed and harvest it simply for the onion greens.

The onion bulb itself is pretty much done growing. All the energy is going into the flower & seeds.

Enjoy the natural beauty of the flower! That is probably your best option at this point.

A bee with a taste for onion.

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Other pollinators (dirt dauber):

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Beautiful Pics Bill!

Onion flowers start out as a bulb on a scape:
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Then the little bulb begins to open:
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Then finally, a beautiful flower:
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The flowers smell faintly of onion and are quite pretty. I hope to harvest some seed from this one; a red short-day onion called Southern Belle.

Outstanding photos.

Onions are biannuals. Meaning that they flower in the 2nd year. Your onion is done. It is producing seeds this year not bulbs.

Every year I miss a few onions during harvest (or get to them too late) and let them grow the following year just go have random big white balls throughout my garden. Kind of pretty and makes the neighbors wonder.

The seeds should be viable for next year, but without knowing the variety you should plant on using them for green onions.