First Step: Prepare the Peppers and Pesto Preheat your grill to medium-high or set your oven broiler to 500°F (260°C). Brush both sides of the halved, seeded bell peppers lightly with olive oil. For the optional arugula pesto, combine arugula, toasted walnuts, Parmesan cheese, and garlic cloves in a food processor, pulsing until finely chopped. Gradually add olive oil until smooth, then season with salt and pepper.

Second Step: Grill or Broil the Peppers Place the peppers cut side down on the grill or a foil-lined baking sheet under the broiler. Cook for about 4-5 minutes per side until the skin chars and blisters but the pepper remains firm. If broiling, turn occasionally for even charring. Remove from heat and season with sea salt and freshly cracked black pepper.

Third Step: Prepare the Bread and Cheese Brush the outer sides of the ciabatta buns with olive oil. Spread pesto evenly on the cut sides of the bottom halves. Layer sliced mozzarella over the pesto. During the final two minutes of pepper cooking, toast the buns cut side up on a grill rack or in a pan away from direct heat until the mozzarella slightly melts and the bread turns golden and crispy.

Fourth Step: Assemble the Sandwich Place two roasted pepper halves or strips on the mozzarella-covered bottom bun. Drizzle with aged balsamic vinegar or a thick balsamic glaze. Add fresh basil leaves if desired. Cover with the top half of the bun, press gently, and slice in half if preferred.

Final Step: Serve and Enjoy Serve your roasted pepper and mozzarella sandwich immediately while warm to enjoy the best texture and vibrant flavors. Pair it with a fresh salad or soup for a complete meal.