It was a huge hit with my American family visiting Italy a few weeks back, and when it happened to complement a mixed platter of artisan cheeses at an Abruzzo inn, the jar of onion jam was scraped clean.
It's a tangy chutney-like preparation, in fact many Italians refer to it as "mostarda di cipolle," a common moniker for sweet and spicy fruit or vegetable relishes. It's an available item in most Italian delis, but I prefer making mine at home.
This particular one is good with goat's milk cheeses, aged and smelly ones, or a humble chunk of Parmigiano. It tends to overpower the cheese flavor, so should be slathered with measure.
2 kg (4.4 lb) red onions
400 gr (14 oz) organic brown sugar
500 ml (2 cups) balsamic vinegar
1 teaspoon mustard powder
Sterilize the designated mason jars (+6 medium capacity) by boiling them in plenty unsalted water for 10 minutes.
Trim away papery outer shells and peel the onions clean. Thinly slice them while holding a chunk of bread in your mouth to prevent tears.
Wilt the onions with 2 tablespoons of water in a large heavy-bottomed saucepan over very low heat. Gently simmer until they become translucent, then add sugar and caramelize them slightly. Stir in the vinegar and mustard powder, cover and keep at a gentle simmer until the jam thickens (about 40 min to an hour).
To test for doneness, pour a drop of jam on a cold plate, if it slides off and is still quite runny, keep cooking until the drop sticks to the plate. Careful not to scorch it, though. Where there is sugar boiling, there is always that one split second of distraction that can cause the whole thing to burn and stick horribly to the bottom of the pan.
Divide the marmalade while still hot into the individual sterilized jars and quickly close with new caps. Turn jars upside down and cover them huddled together under a warm fleece or a woollen blanket. This assures vacuum and pasteurization. For safety, after a few hours, check that the vacuum valve is depressed on the lids. If the valve snaps, boil the jam jars in boiling water for 10 minutes.
When completely cooled down, store in a cool, dark place. Shelf life is about 6 months. Once opened, store in the refrigerator, and consume within the week.