Cook the pasta: Bring a large pot of salted water to a boil. Add the pasta and cook for about 8 minutes, or according to package directions, until al dente.
Cool the pasta: Drain the pasta in a colander and rinse briefly under cold running water to stop the cooking process. Drain very well and allow it to cool completely.
Prepare the onion: Place the thinly sliced red onion in a small bowl. Add a little lemon juice and a pinch of salt. Gently massage the onion for a few seconds, then let it sit while you prepare the remaining ingredients.
Prepare the salad: Add the diced tomato, sliced olives, corn, and drained tuna to a large mixing bowl.
Add the onion: Drain or gently squeeze any excess liquid from the softened onion and add it to the bowl.
Combine: Add the cooled pasta, chives, and olive oil. Gently toss until all the ingredients are evenly distributed.
Season: Taste and add more salt, olive oil, or lemon juice if needed.
Chill and serve: Cover the bowl and refrigerate before serving. Serve chilled, garnished with extra chives, sliced olives, or fresh tomato if desired.
