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.