Place all of the marinade ingredients in a ziploc bag and add the chicken. Seal it, pressing out as much air as possible and place in the fridge for 30 minutes or up to 1 day.
Preheat the oven to 350 degrees.
Butterfly the chicken by slicing it open, leaving the back connected like a butterfly.
Place mozzarella, pesto, spinach, tomatoes and olives inside and seal closed with toothpicks.
Whisk together the vinegar, brown sugar and lemon zest.
Heat a pan to medium high heat with a drizzle of olive oil.
Once shimmering, place the chicken in the pan and turn to medium low heat.
Cook for 4 minutes and once browned, turn over.
Add the balsamic and brown sugar mixture and cook for 5 minutes.
Place in the oven for 15-20 minutes or until cooked through.
Cover with the pan drippings and serve!