Question
Is there a product out there which I could spray on my onion grass to get rid of it?
Sincerely,
Ron

Answer
This is the first time I have heard about onion grass and all the references about it seem to come from Australia. Are you in Australia?
Regretfully, I must decline to answer your question because of my ignorance.
"Glyphosate", the active ingredient in many non-selective weed killers may kill it but please be careful because it will kill everything green that it touches.