I made a raised row about a foot and a half wide and covered it with 3-foot wide black plastic mulch.
Then I made small slits about 3-inches apart in both directions and pushed an onion bulb through each slit.
Two bags of onion sets of about 80 bulbs each covered about 7 feet of row.
It has worked very well keeping the weeds down.
I don’t think the bulbs are overcrowded, but next year I think I should try spacing them either 4 or 6 inches apart.
Also, although they seem to be getting enough water, I think I should make a depression down the middle of the row to collect water and allow it to seep through the slits.
Edit: 3 inches apart is definitely too close together, the onions are barely the size of golf balls.