I've done it with one of the landscape cloth type mulches. It really kept the weeds down and I was really happy with that part of it. I watered with soaker hoses laid down on top of the landscape fabric (which also had a couple of inches of old hay on top of it), with one hose in between every two rows of onions. It worked well enough. At harvest time, you either have to cut the holes in the mulch material larger so you can pull the onions out through the holes (my holes were just X's cut in the mulching material, or lift the whole sheet of mulch so you can pull the onions. You still have to deal with weeds that pop up directly beside the onions, but that's nothing compared to how many winter seeds, in particular, sprout in newly planted onion beds.

Bruce Frasier wrote about using plastic mulching long ago in one of the Dixondale newsletters, and if he says it can be done successfully, then it can be done. I think lots of commercial growers do it.

I only did it for a handful of years. I found that I prefer to use a pre-emergent (corn gluten meal) at planting time and then just hand-weed what little bit of weeds sprout. By the time the pre-emergent is losing its effectiveness, my onions have made some good growth and I just put another layer of mulch on. My first later of mulch is usually about a half-inch of compost. Then I put on about an inch of old, spoiled hay. Then, as the onions enlarge, I add a layer of grass clippings. If the mulch thins out or weeds stop popping through, I add more mulch. I like using organic matter more than plastic because the organic matter decomposes and enriches the soil. Having said that, though, there's certainly nothing wrong with using plastic mulch. It works fine.

I am not sure how well it would work in a very wet year. Since I have heavy clay, I always worried that the plastic might hold in too much moisture in a wet year, but we don't often have wet years here at our place, so I worried for nothing. I did love how the mulching material reduced erosion. I garden on a slope. There is not much flat land in my garden at all, so erosion can be an issue and the plastic certainly prevented much of that from happening.