I always put down a piece of cloth or something to catch what blows away. Not because I care about the chaff, but because sometimes when the wind blows (or my fan that was hooked up today), it blows a bit too hard. Sometimes I lose seed and the cloth makes it easy to pick up. The other required piece is an old pillowcase. I put the seed heads in and stomp on them or if the seeds are delicate I use my hands to crush them all up. The mustard seeds can handle the abuse of being stomped on. The yellow mustard was easy to separate as the seed pods just shatter. The brown ones on the other hand were a pain in the butt. The seed heads just didn't want to open. And when they did, there was a thin layer separating the two side that wouldn't come off. So I'd get half the seeds from a pod. In addition the brown pods had some germination inside the pods. The germinated ones don't separate easily from the seed. I wonder if it will affect the quality of the mustard I make later. I hope not.
And I have to say, using a fan is so much easier than relying on the wind. The wind gusts so can really blow the seed where you don't want it. Or it stops so the chaff doesn't get blown away. But a fan can be controlled. It is the first time I've used one to winnow my seeds.
And as a side note. The onion seed is from some hybrid plants. So who knows what I'll get. I don't have room to let enough bunching onions go to seed to maintain genetic viability. But the seed from a hybrid is variable enough that it will grow well. I just don't know what I'll get. I figure it is a bunching onion. So who really cares?