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Make the sherry onions up to 2 days before, cover and chill. Bring up to room temperature to serve.
For the sherry onions, heat the butter and olive oil in a nonstick frying pan and fry the onion, uncovered, over a low heat for 30 minutes until tender and looking sticky, stirring every so often.
Preheat the oven to 180°C, fan 160°C, gas 4. Add the vinegar and sugar to the onions, and season well. Scrape any bits from the bottom of the pan, then add the sherry, and simmer until the liquid has evaporated. Remove from the heat.
Remove all paper packaging from the cheese and put it in a small, snugly fitting baking dish, or back in the wooden box it came in if it’s stapled together (but not if it’s glued). Mix together the extra-virgin olive oil and rosemary and brush a little over the cheese. Cook for 10-12 minutes until the centre looks wobbly.
Meanwhile, slice the ciabatta into 5mm-thick slices and transfer to a baking sheet. Brush with the remaining rosemary oil, season and bake for 5-10 minutes until golden and crisp. Serve the cheese with the onions piled on top, and lots of toasts to dig in.