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Heat oven to 180C/160C fan/gas 4. Slice the root off each onion to create a flat bottom, then carefully peel, leaving the onion whole. Cut a deep cross in the top of each, then poke in a sprig of rosemary, dot with butter and place in a roasting tray. Pour a little of the balsamic vinegar over each one and season. Wrap the tray in foil and bake in the oven for 30 mins. Meanwhile, mix together the mustard and sugar and set aside.
Remove the tray from the oven, take off the foil and spoon a little of the mustard mixture onto each onion. Return to the oven for 15 mins, uncovered, to caramelise.