In a large mixing bowl, combine the cooked chicken, crumbled bacon (if using), diced tomatoes, celery, red onion, and green onion.
In another bowl, whisk together the mayonnaise, apple cider vinegar, dried parsley, dried dill, chives, garlic powder, onion powder, salt, and pepper.
Pour the dressing over the chicken mixture and gently toss until everything is well-coated.
Butter one side of two slices of bread. Place one slice butter-side down on a skillet.
Scoop a generous portion of the chicken salad mixture onto the slice in the skillet, and then top with another slice of bread, butter-side up.
Grill over medium heat until the bread is golden brown and the cheese is melted, flipping once to ensure even grilling.
Cut in half to serve and enjoy this fabulous comfort food!