Leave a few inches of green onion above the bulb when you cut material away from your store-bought scallions for cooking. Take care to protect the roots extending out from the bulb as well.
Set the remainder of the scallions into a jar and fill the jar with water until the white bulb and base of the scallions are half-submerged. Leave some space around the scallions, and do not stuff the jar with them.
Place the jar on your windowsill or a counter or shelf where it will get plenty of sunshine.
Pour out the water in the jar every other day and replace it with fresh water. Cut away freshly regrown scallions as needed, but take care to leave each individual plant with at least a few inches of green onion above the bulb.