Halve or roughly chop the tomatoes (depending on size) and put them in a bowl. Season well and add the garlic, capers, avocado and onion, and mix well. Set aside for 10 mins.
Meanwhile, tear or slice the ciabatta into 3cm chunks and place in a large serving bowl or on a platter. Drizzle with half the olive oil, half the vinegar and add some seasoning. When ready to serve, pour over the tomatoes and any juices. Scatter with the basil leaves and drizzle over the remaining oil and vinegar. Give it a final stir and serve immediately.