Cook the pasta: Boil pasta in salted water according to package directions until al dente. Drain and rinse under cold water to cool.
Make the dressing: In a large bowl, mix mayonnaise (or Greek yogurt), Dijon mustard, lemon juice, salt, and pepper.
Add mix-ins: Stir in tuna, celery, onion, peas, and corn.
Combine: Add cooled pasta and toss until everything is well coated.
Chill: Cover and refrigerate for at least 30 minutes to let the flavors meld.
Serve: Garnish with parsley or dill before serving. Enjoy cold or slightly chilled.