Prepare the Chicken: If not using pre-cooked chicken, cook 1 lb boneless, skinless chicken breasts by boiling, poaching, baking, or grilling until cooked through (internal temperature 165°F/74°C). Once cooled slightly, shred or dice the chicken into bite-sized pieces.
Cook the Bacon: Cook the bacon until crispy. You can do this in a skillet, in the oven, or microwave. Once cooked, transfer to a paper towel-lined plate to drain excess fat, then crumble into small pieces when cool enough to handle.
Prepare Vegetables: Halve or quarter the cherry/grape tomatoes. Finely dice the celery and red or green onion.
Make the Dressing: In a medium bowl, whisk together the mayonnaise, Greek yogurt (if using), Dijon mustard, lemon juice, garlic powder, onion powder, black pepper, and a pinch of salt. Taste and adjust seasonings as needed. If using dried dill or ranch seasoning, mix it in now.
Combine Ingredients: In a large mixing bowl, combine the shredded or diced chicken, crumbled bacon, halved tomatoes, diced celery, and diced onion.
Dress the Salad: Pour the prepared dressing over the chicken mixture. Stir gently until all ingredients are well coated.
Chill (Optional): For best flavor, cover the bowl and refrigerate the chicken salad for at least 30 minutes to allow the flavors to meld.
Add Lettuce and Serve: Just before serving, if desired, gently stir in the shredded romaine lettuce to maintain its crispness.