Preheat oven to 350℉. Line a rimmed baking sheet with aluminum foil and set it aside.
One at a time, cut each chicken breast in half horizontally through the center, creating two thinner pieces.
Set a large skillet over medium-high heat. When the pan is hot, add 1 tablespoon oil. When oil begins to shimmer, add the chicken and cook for 3-5 minutes on each side, flipping as sides get brown to ensure they don't burn.Set cutlets aside on a plate once cooked
In small bowl, mix olive oil with salt and pepper. Use basting brush to spread olive oil mixture on inside of bread slices.
Place bread onto frying pan for 30 seconds or until grill marks can be seen.
Remove bread and place onto baking sheet
Spread pesto lightly on each piece of bread
Layer chicken, spinach, tomato, and mozzarella onto bread
Drizzle balsamic vinegar over sandwich toppings
Bake in preheated oven for 7-10 minutes, until cheese begins to melt.