Cheese melt with apple & red onion relish

Heat the olive oil in a frying pan and add the butter and onion. Cook over a medium-low heat, stirring often, for a few mins. Add the apple slices and cook until softened. Sprinkle in the sugar and cook until the onion and apple turn a rich, golden brown. Remove from the heat and stir in a few drops of balsamic vinegar (if using). Preheat the grill to high. Toast the slices of farmhouse loaf or ciabatta bread on one side only. Heap the onion and apple mixture onto the untoasted sides. Top with the Cheshire cheese and a sprig or two of rosemary or thyme. Season with freshly ground black pepper. Toast under the grill for a few moments until the cheese starts to melt and bubble. Serve immediately.