I don't think there are any commercial hydro onion farms. It is possible to grow them in hydro, but the value of the crop is low compared to other options. So there would be no standard or "correct" answer to your question in the sense of commercial growing. Maybe the link below can show you some successful experiments.

However, green onions, aka chives, would be more realistic as a hydro crop. Copy and paste this link for a good pic: http://fourstarhydroponics.com/wp-content/uploads/2012/04/Chives.jpg

Here is a link that might be useful: Google Images search for hydroponic onions

Wow what a great photo, I agree root crops do not always offer the best in hydroponics but leaf crops including chives are very good. I have also had good success with tomatos & Chillies , but crops with invasive root growth eg Kang Kong can upset and block drainage systems.
One of my friends in Australia who grows parsley as a commercial crop on a large scale eg for restaurants etc, has his crop growing in sawdust, with non return nutrients , however he has drip holes in the bottom of his plastic sheet troughs holding the parsley , and in the ground below the troughs he has onions growing in the soil, so he has a hydroponic crop and at the same time has a soil crop fed by the hydroponic nutrients.

I want to grow herbs, by hydroponis, and use my own pvc pipes and make my own nutrients, I want to be the first one to do this in my group and I have no experience at this, I grow herbs and veggie in soil but I want to teach my offenders something new and I want us to win a prize, but the catch is we have go cheap cause it all come out of my pockets, or from stuff I can round up. Please help.

Here is a link that might be useful: GardenWeb