Welcome to the Bunnings Workshop community @TimD. It's fantastic to have you join us and many thanks for your question.
There are only two ways I know to be effective in removing onion weed. Applying Yates Zero Weed Killer Super Concentrate 490 onto them with a paintbrush or using a Yates Zero Weeding Herbicide Applicator Brush and digging the bulbs out by hand. There are no selective weed sprays that I know of that will not kill the grass as well. You might need a couple of applications of the Glyphosate as the bulbs can re-shoot. If you decide to dig them out by hand ensure you collect all the bulbs when removing.
You might like to read through these previous discussions on onion weed: