Those are aphids. Are you growing the onions indoors? If they are bulbing onions (not scallions, where you would be eating some of the green), then you could spray them with insecticidal soap. Outside, I would just hose them down.

Growing them outdoors! And they are scallions...

Then just spray them off with a strong stream of water. Or you can smoosh them with your fingers if you can tolerate the 'ick' factor.

You could also dust them with powdered gypsum if the spraying-off doesn't work.