Trim the top and bottom of the onions minimally, otherwise they will break up during cooking.
Set the oven to low, 140°C/275°F/Gas 1.
Take a pan wide enough to hold the onions in a single layer and melt the butter in this over a medium flame. As soon as this starts to brown, add the sugar and half the vinegar. Add the onions and stir to coat.
This dish is best served warm. If you try to serve it hot, the sugar content may scald your guests’ palates.