Cook the pasta. Bring a large pot of salted water to a boil.
Add pasta and cook until just al dente. You want a firm bite so it holds up in the salad.
Cool it down. Drain the pasta and rinse under cold water to stop the cooking. Shake off excess water, then spread on a tray for a few minutes to dry.
A little drying helps the dressing cling.
Make the dressing. In a large mixing bowl, whisk together mayonnaise, Greek yogurt, Dijon, lemon zest, and 1–2 tablespoons of lemon juice. Add olive oil, a pinch of salt, black pepper, and garlic powder if using. Taste and adjust the lemon and seasoning.
Prep the add-ins. Flake the drained tuna with a fork.
Chop the onion, celery, and parsley. Halve the cherry tomatoes and drain the sweetcorn if using canned.
Toss gently. Add the pasta to the bowl of dressing and stir to coat. Fold in tuna, sweetcorn, onion, celery, tomatoes, and parsley.
Add capers or gherkins if you like a briny note.
Season and chill. Taste and add more salt, pepper, or lemon juice as needed. Cover and refrigerate for at least 30 minutes so the flavors mingle.
Serve. Give it a quick stir before serving. If it seems a bit tight after chilling, loosen with a splash of olive oil or a spoonful of yogurt.