Onions may be multilayered, but regrowing them is actually quite simple. Slice the bulb so you have a one-inch sliver of the root end. Let that piece dry for a day or two, until the edges start to curl, then place it in the soil, cut side up. Cover the onion piece with about an inch of dirt and water well. Green shoots will start to appear, which you can harvest like spring onions. You can also allow these to continue growing into new onion bulbs, which will be ready to harvest in 90 to 120 days.