Drill holes across it as shown, and put the onion bulb sets (they're tiny onions, about the size of a quarter) into them; fill with soil thru the top, and do the next layer, rinse repeat until full. They will sprout through the holes.
Indoor Vegetable Gardening: 37 Edibles You Can Grow Indoors In The Winter - As a prepper, one of the essential skills is for you to be able to sustain yourself and have food available to you the whole year, if and when you need it.
#3. Putting a potato pot with cut sides inside another one makes it easier to lift the plants out for harvesting