Soak chopped red onion in cold water while you prepare the rest of the salad.
Using a medium bowl, place tuna and all the juices from the cans in the bowl; break the chunks of tuna apart with a fork; the tuna will absorb the liquid.
Add the remaining ingredients to the tuna; toss to mix so everything is covered with dressing.
Serve immediately or cover tightly and store in the refrigerator until ready to serve.