In a medium bowl, combine the drained tuna, mayonnaise, Greek yogurt, lemon juice, and Dijon mustard. Mix until well blended.
Add the chopped green onions, celery, cucumber, parsley, garlic powder, salt, and pepper. Stir until all ingredients are evenly incorporated.
Taste and adjust seasoning if necessary. If you prefer a creamier texture, add more mayonnaise or Greek yogurt.
Chill the salad in the refrigerator for at least 30 minutes to allow the flavors to meld.
Serve on a bed of lettuce leaves or as a sandwich on whole grain bread.