Prep the bread: Split the ciabatta rolls. Brush the cut sides lightly with olive oil.
This helps the bread crisp and adds flavor.
Toast: Place the ciabatta, cut side down, in a skillet or on a griddle over medium heat for 2–3 minutes until golden. Alternatively, toast under the broiler for 1–2 minutes. Watch closely to avoid burning.
Warm the chicken: In the same skillet, add the chicken with a teaspoon of olive oil.
Season with salt, pepper, and red pepper flakes if using. Cook 2–3 minutes until heated through.
Add pesto: Reduce heat to low, spoon 2 tablespoons pesto over the chicken, and toss to coat. This blooms the flavors and prevents a dry sandwich.
Layer the cheese: Place mozzarella on the bottom halves of the toasted ciabatta.
Return to the skillet or broiler just until the cheese starts to melt.
Build the sandwich: Pile the warm pesto chicken over the melted cheese. Add tomato slices and a handful of arugula. Drizzle with a little balsamic if you like.
Finish with pesto: Spread the remaining pesto on the top halves of the ciabatta.
Close the sandwiches and press gently.
Serve: Slice in half if using large rolls. Eat warm for the best texture and flavor.