Cut the tomatoes into bite-sized chunks and place in a colander. Sprinkle over a little sea salt and leave to drain for 5-10 mins. Transfer to a salad bowl.
In a small bowl, add the chopped capers, olive oil, vinegar and garlic and mix thoroughly. Pour over the tomatoes
Add the sliced onion to the bowl, toss together and set aside for 30mins to marinate.
Stir the the tomato and onions together, scatter the basil and parsley over the salad and serve.