Begin by skinning the tomatoes. Bring a small saucepan the boil and add the tomatoes for 30 seconds. Remove from the pan and plunge into ice-cold water.
Peel the skins off the tomatoes, then cut into quarters and scoop out the seeds with a spoon. Cut into small dice and transfer to a large mixing bowl.
Add the remaining ingredients to the bowl and mix well. Serve immediately or refrigerate until ready to serve.