Heat the butter in a large frying pan over a moderately high heat. When melted, add the onions and salt. Cook the onions until a deep golden colour, stirring regularly. Add the balsamic vinegar and cook for a further 5 minutes until all the liquid has evaporated and the onions are sticky.
Mix together the flour, yeast, sugar, salt, oil and 275ml of warm water in a bowl to form a soft, smooth dough that leaves the sides of the bowl. This can be done by hand or using a stand mixer fitted with a dough hook. Knead for about 10 minutes, until dough is smooth.
Lightly oil the inside of a bowl, place the dough into it and cover with lightly oiled clingfilm. Leave in a warm place for about 1 hour, or until the dough has doubled in size.
Tip the dough out onto a floured worktop and roll it into a large rectangle, about 1cm thick. Spread the onion jam over the dough, leaving a 4cm border on one long edge of the dough. Top with the olives and thyme leaves. Carefully roll the dough in on itself, leaving the border exposed. Brush the border with warm water then fold over the top of the dough. Turn the bread over so that the border seal is underneath. Cut the roll into 8 slices and place each piece cut-side up on an oiled baking tray. Cover with clingfilm and place in a warm place to prove for 30 minutes.
Preheat the oven to 170°C/fan 150°C/gas mark 3.
Remove the clingfilm and bake the rolls for 16-20 minutes until golden and cooked through. Serve warm.