Preheat the oven to 190C/170 Fan/Gas 5.
Put the tomatoes and onion in a roasting tray, drizzle with ½tbsp of the oil and season with pepper. Roast for 20 mins or until soft. When cool enough to handle, remove the stalks.
To make the bruschetta bases, drizzle the ciabatta with the remaining oil and griddle in a dry frying pan until crisp and toasted.
To assemble, pile the tomatoes and onions on the ciabatta, sprinkle with the oregano and rocket, then drizzle over the balsamic glaze.