For the onions, preheat the oven to 180°C. Place them into an ovenproof dish with the butter, pepper and balsamic. Toss to combine, cover tightly with foil and bake for 1 hr 15 minutes. Mix the onions around with tongs every 15 minutes so that they cook evenly. Cool completely before using. This can be done 1 or 2 days in advance.