Cook the orecchiette in a pan of boiling salted water following the pack instructions, or until al dente. Drain the cooked pasta in a colander and then rinse under running cold water until completely cool.
In a large bowl, combine the cold pasta, tomatoes, onions and tuna. Pour over the olive oil, toss and season with a little salt and some freshly ground black pepper. Serve with a sprig of parsley, if you like.