In a large bowl, combine the shredded chicken, Caesar dressing, grated Parmesan cheese, garlic powder, and black pepper. Mix until well combined.
Toast the sourdough bread slices until golden brown.
On two slices of toasted bread, layer the chopped romaine lettuce and the chicken mixture evenly.
Top with halved cherry tomatoes and croutons if using.
Place the remaining slices of toasted bread on top to form sandwiches.
If desired, sprinkle extra Parmesan cheese on top before serving.
Cut the sandwiches in half and serve immediately.