Keep cool and dry, preferably somewhere with a bit of air movement so they can breathe.
Prep as you would any normal onion. They are quicker to peel and slice if topped, tailed and halved first. Any trimmings are a welcome addition to a stock pot.
Slice thinly and eat raw in salads and slaws. Alternatively, roast them whole: cut a deep cross into each onion, almost down to the root but not quite. Force in some softened butter and a few sprigs of thyme. Rub the onion with oil and season with salt and pepper. Place your onions into a snug-fitting roasting tray, add a dash of water, cover well with foil and bake at 200°C for 40 mins before removing the cover and finishing for another 10 mins. They should be tender and collapsing.