Remove the root end of the onion with at least an inch of green still attached. Chop the rest of the green tops and put into bags in the freezer for later use.
Fill small pots with potting mix. Poke holes 1/2 inch apart in the mix with your finger deep enough to cover the white part of the onion bulb. Place the onion bulb root-down into the hole and firm the soil around it.
Keep the onions moist but not wet so they don't rot. Fertilize once a month with fish emulsion or compost tea. Clip the tops as needed, and remember to leave 1 inch of green above the ground.
Remove the onions from the pot and separate them when they get crowded. Start more in pots, share with friends or chop up and freeze for later use.