Preheat a grill or stovetop skillet to medium-high heat.
Brush the chicken breasts with olive oil and season both sides with salt, pepper, and garlic powder.
Cook the chicken on the grill or skillet for 6-8 minutes per side, or until fully cooked and the internal temperature reaches 165°F (74°C). Remove from heat and let rest for 5 minutes before slicing into thin strips.
Toast the ciabatta bread slices lightly in the oven, on a grill, or in a toaster until golden and crisp.
Spread 1 tablespoon of pesto on one side of each bread slice.
Layer the sliced chicken on two of the bread slices, followed by mozzarella slices, fresh spinach, and tomato slices.
Top with the remaining bread slices to form sandwiches.
Optionally, press the sandwiches in a panini press or skillet for 1-2 minutes to melt the cheese further.
Slice each sandwich in half and serve warm.
Notes
You can substitute ciabatta bread with sourdough or whole-grain bread for a different flavor. Add avocado slices for extra creaminess.