Whisk together mayonnaise, Greek yogurt, lemon juice, garlic powder, salt, and black pepper in a small bowl until smooth.
In a large mixing bowl, combine diced chicken, bacon crumbles, halved cherry tomatoes, and sliced green onions.
Fold the dressing into the chicken mixture until all components are thoroughly coated.
Add the chopped Romaine lettuce to the bowl and toss gently to incorporate without wilting.
Serve immediately or store in an airtight container for up to 24 hours.