In a medium bowl, combine the cooked chicken, Caesar dressing, grated Parmesan cheese, Dijon mustard (if using), garlic powder, and black pepper. Mix thoroughly for about 2-3 minutes until the chicken is well-coated in the creamy dressing.

Gently fold in the chopped romaine lettuce and cherry tomatoes (if using) into the chicken mixture.

Preheat your oven to 350°F (175°C). Slice the croissants in half horizontally and place them on a baking sheet, cut-side up. Toast in the preheated oven for about 5-7 minutes.

Remove the croissants from the oven and spoon the creamy chicken mixture onto the bottom half of each croissant.

Place the top half of the croissant over the filling and press down lightly. Serve immediately.