1) In a large bowl, add the tuna, reserving the olive oil in a separate small bowl. Break the tuna into bite-size pieces with a large fork. Add the beans and capers.
2) Into the bowl of olive oil, add the red wine vinegar. You should have one part vinegar to two parts oil. Add more extra virgin olive oil if necessary. Season with salt and pepper.
3) Pour the dressing onto the tuna, bean and caper mixture and allow the flavours to infuse while slicing the vegetables. Add the onion and tomatoes to the tuna mixture and toss gently.
Place the rocket on a large decorative platter and top with the tuna mixture. Tear fresh basil leaves over the top and serve immediately.