In a medium bowl, whisk together the mayonnaise, Greek yogurt, Dijon mustard, lemon juice, water, dill, garlic powder, salt, and black pepper until smooth and well combined. Finely chop the parsley and stir it in. Set the dressing aside—I like to make this first so the flavors can meld while you prepare the other components, which deepens the overall taste of the salad.

Bring a large pot of salted water (about 1 tablespoon salt) to a boil. Add the green beans and cook for 2-3 minutes until bright green and tender-crisp. Remove the beans with a slotted spoon and immediately transfer them to a bowl of ice water to stop the cooking process and preserve their color. Once cooled (about 5 minutes), drain and cut the beans into 1-inch pieces if not already cut. In the same pot of boiling water, cook the elbow pasta according to package directions until al dente, then drain and set aside.

While the pasta cooks, dice the English cucumber into bite-sized pieces, halve the cherry tomatoes lengthwise, thinly slice the celery stalks, and finely dice the red onion. Drain the canned tuna and break it into chunks. Keep all prepared ingredients separate until assembly for easier mixing.

In a large serving bowl, combine the cooked pasta, cooled green beans from Step 2, and all the prepared vegetables from Step 3. Pour the dressing from Step 1 over the mixture and toss gently but thoroughly until all components are evenly coated. I find that letting the salad sit for 10-15 minutes before serving allows the pasta to absorb some of the dressing and the flavors to marry beautifully, though you can serve it immediately if preferred.