In a large mixing bowl, whisk together Greek yogurt, mayonnaise, Dijon mustard, and lemon juice until smooth and creamy. Add onion powder, celery seeds, salt, and pepper and mix well.

Fold in chopped cooked chicken breast, dried cranberries, diced celery, and sliced green onions into the dressing mixture until all ingredients are well-coated.

Cover with plastic wrap and refrigerate for at least 1 hour to allow flavors to meld.

Prepare burger buns by laying out the bottom halves and optionally toasting them for 2-3 minutes until golden.

Spread a layer of fresh lettuce on the bottom half of each bun, then generously scoop the chilled chicken salad onto the lettuce.

Layer thin slices of tomato and red onion on top of the chicken salad, then close the sandwiches with the top halves of the buns.

Cut the sandwiches in half for easier handling and serve immediately.