Pound chicken to an even 1/2-inch thickness for quick, even cooking.
Season chicken with olive oil, lemon juice, salt, pepper, garlic powder, and Italian seasoning. Toss to coat and rest 10 minutes.
Heat a grill pan or skillet over medium-high. Cook chicken 4–5 minutes per side until 165°F. Rest 5 minutes, then slice.
Warm a panini press or large skillet on medium. Spread pesto on cut sides of the ciabatta.
Layer mozzarella, sliced chicken, tomato, red onion, and spinach on the bottom halves. Cap with the tops.
Lightly butter the outsides of the sandwiches.
Press or cook 3–4 minutes per side until bread is crisp and cheese melts. Slice and serve with balsamic glaze.