Acquaviva Red Onion

This is a local ecotype of red onion grown at Acquaviva delle Fonti and in the neighbouring towns of the Murgia plateau, south of Bari. It carries no EU designation: it is listed among Puglia's traditional agri-food products and protected as a Slow Food Presidium, which here mostly guarantees that the seed is locally selected rather than bought in.

The bulb is the signature. It is markedly flattened: a disc 10 to 15 centimetres across and only a few centimetres tall, not a sphere, with violet-red outer skins and white flesh veined with pink. Normal weight is 200 to 500 grams, and the best bulbs pass a kilo. Sliced across, it gives rings the size of a saucer, thick and fleshy enough to hold together on a plate.

The second characteristic is sweetness. Sugar content is high and the sulphur compounds that make onions sting are low, so cutting one rarely brings tears and eating it raw leaves none of the harsh aftertaste of a standard onion. In Acquaviva the onion is not a base for a sofrito: it is a vegetable in its own right, served the way you would serve a tomato.

Seed selection stays with the growers. Each year the best bulbs are held back, replanted the following season to run to seed, and the seed is saved from those. That is why the onion has stayed as it is, and also why production remains small.

Sweet and juicy rather than sharp, with a clean mineral finish and only a faint onion bite at the end. The closest reference for an American palate is a Vidalia or a Walla Walla; for a British one, a Roscoff pink onion. It is noticeably sweeter than a supermarket red onion and much less aggressive.

Raw, the texture matters as much as the flavour: the rings are thick and crisp, and they snap rather than bend. That is why the local way of eating it is simply sliced with salt, oil and oregano, because anything more elaborate gets in the way.

Roasted whole it goes somewhere else entirely, collapsing into something jammy and almost caramel-sweet. Long-cooked in a pan it sweetens fast and, because of those sugars, catches and browns sooner than a yellow onion would.

Raw first, because that is the whole point. Cut half-centimetre slices across the bulb, dress with salt, extra virgin olive oil and dried oregano, and eat with bread. Add tomato and mozzarella and you have the summer salad of the Murgia; put it alongside boiled fava purée and chicory and you have the region's oldest plate.

Roasted whole is the second use and the easiest. Leave the outer skins on, sit the bulb in a dish with a little oil, and give it ninety minutes at 180 degrees. It comes out collapsed and caramel-sweet: split it open and dress with oil and vinegar.

The local set piece is the calzone di cipolla: a kilo and a half of sliced onion stewed slowly in olive oil until it falls apart, mixed with black olives, salted anchovies and raisins, then sealed between two discs of dough and baked. It is eaten warm rather than hot.

Thick slices also grill well (brush with oil, turn once) and stew into a sweet-and-sour agrodolce with vinegar and a spoon of sugar to serve with cheese.

One practical warning: the sugars that make it good raw make it catch quickly in a hot pan. Keep the heat low and add a spoon of water as soon as it starts to colour.

The honest note first: a sweet onion holds more sugar and more water than a standard one, and that is exactly why it keeps less well. Yellow and white storage onions run a year; this one manages three or four months, which takes a July harvest to about November. A braid hung in a cool, dark, airy room is the best arrangement, and the braid is functional: the air moving between the bulbs keeps the tunics dry. Whole bulbs do not go in the refrigerator, where the damp settles at the neck, and they do not go next to potatoes, because each pushes the other towards sprouting.

A cut half goes into a closed box in the fridge for about two days; after that the exposed rings dry, darken and turn sharp-smelling. Over months the bulb loses water through the papery skins, the flesh tightens, and eventually a shoot pushes up the middle. Keeping the crop past its natural span means changing it: pickled sweet and sour, cooked down with sugar and wine, or sliced raw and frozen for six months, fine for stews and calzoni but never crisp again.

Shape settles it before colour. An Acquaviva onion is flattened, wide and low, with a diameter at least twice its height, so a round or teardrop bulb is not this onion however red the skin. Size follows: good ones weigh 200 to 500 grams and often go past a kilo, so an 80 gram red onion is another variety. The outer tunics are wine red going violet, dry and papery, but the flesh underneath is white with pink rings rather than red to the core; uniformly purple flesh usually means a northern European onion.

Then use your eyes and nose. Cut in half it should not attack you: stinging eyes and a hard pungency mean the sweetness is missing. The bulb should feel firm with a tight, dry neck, since a thick, soft or still-green neck comes from an early lift. For an American shopper the closest reference point is a Vidalia, flat and sweet in the same way though yellow rather than red; cipollini are the same shape at a quarter of the size. Crates and braids should name the grower or cooperative, because cipolla rossa pugliese says nothing.

Realistically, in Puglia and nowhere else. This is not an export onion: it holds less well than a storage variety, travels badly, and the total area planted runs to a few hundred hectares. In season it is on market stalls across the province of Bari at 1.50 to 3 euro per kilo loose, or 3 to 5 euro per kilo braided, since the braiding is all hand work. A twenty-bulb braid weighs about four kilos and costs 12 to 20 euro.

Elsewhere in Italy it appears in specialist greengrocers in summer at roughly double that. Abroad it is a rarity: an Italian deli might get a few braids in July at 8 to 14 dollars a pound in the US or 10 to 18 pounds a kilo in the UK.

What does travel is the seed, sold by several Italian seed houses. If you have a vegetable garden, growing it is easier than finding it: sow in a seedbed in autumn, transplant in late winter, lift in June.

When buying, judge by shape rather than colour: broad and flattened, at least twice as wide as tall, with a tight dry neck. A round bulb with red skin is a different onion.

Acquaviva delle Fonti itself, where the onion calzone is sold in the bakeries of the old centre all year and the town's onion festival fills the streets at the end of summer, though dates move, so check before travelling.

June and July are the better months for the raw product, when the new braids appear on the stalls and the bulbs still have a soft neck. That is also when trattorie across the Murgia put it on the table sliced with tomato and oregano.

The natural circuit takes in Sammichele di Bari, Gioia del Colle and Santeramo in Colle, which cook the same way, and pairs the onion with the rest of the plateau's larder: fava purée with chicory, Altamura bread, a piece of caciocavallo.

In Bari, the market stalls in the centre carry it in season, and the trattorie of the old town serve it raw in salads and stewed into focaccia.