Cook the pasta in boiling salted water according to package instructions until al dente—this usually takes about 8-10 minutes. Drain, then rinse under cold water to stop the cooking process.

In a bowl, mix together the mayonnaise, Dijon mustard, lemon juice, salt, and pepper until smooth.

In a large mixing bowl, combine the cooled pasta with the drained tuna, halved cherry tomatoes, diced cucumber, thinly sliced red onion, and chopped parsley.

Pour the dressing over the pasta mixture and toss gently until well-coated.

Cover the salad and chill it in the refrigerator for at least one hour before serving.