Heat your grill or grill pan to a medium-high temperature.
In a mixing bowl, combine garlic powder, onion powder, Italian herbs, salt, and pepper.
Generously coat both sides of the chicken breasts with the seasoning blend.
Drizzle olive oil over the seasoned chicken.
Grill the chicken for approximately 6-7 minutes on each side, or until the internal temperature reaches 165°F.
Remove the chicken from the grill and allow it to rest for about 5 minutes.
Grill the ciabatta or sourdough bread slices until golden brown.
Spread basil pesto on each slice of toasted bread.
Slice the rested chicken and layer it atop two of the pesto-covered slices.
Add mozzarella cheese, arugula or spinach, and fresh tomato slices.
If desired, drizzle with balsamic reduction.
Top with the remaining slices of bread to complete the sandwiches.
Serve immediately.