Bring a large pot of salted water to a boil. Add the elbow macaroni and cook according to package instructions until al dente, about 7-9 minutes.
Once cooked, drain the pasta in a colander and rinse under cold water to cool it down quickly. Set aside.
In a large mixing bowl, combine the drained tuna, cherry tomatoes, cucumber, and red onion.
In a separate bowl, mix together the mayonnaise, Dijon mustard, olive oil, lemon juice, salt, and pepper until smooth.
Add the cooled pasta to the mixing bowl with the tuna and vegetables. Pour the dressing over the mixture and gently toss until everything is well combined.
Taste and adjust the seasoning, adding more salt and pepper if needed.
Serve immediately, garnished with fresh parsley if desired, or refrigerate for 30 minutes to let the flavors meld together before serving.