Sweet spring onions — not to be confused with scallions — are simply onions that are pulled in the spring to thin out onion beds. They look like large scallions with thick stalks and bulbs that are 2 to 3 inches across. About half of the stalks should be removed, but don’t throw them out. Tuck them in the freezer for stock. Combine the rest of the spring onions with watercress, peas, and parsley in a sprightly soup with a pretty green color.