Preheat the oven to 350F.
Cut each chicken breast lengthwise to make a pocket, make sure not to slice it all the way through.
In a small bowl, mix 3 tablespoon olive oil, salt, pepper, oregano and garlic. Brush this mixture inside and outside of the chicken breasts.
Spread pesto inside all chicken breasts and stuff with basil, tomato and Stella® Fontinella® Basil Pesto Cheese slices. Seal each chicken breast using toothpicks.
Heat the remaining two tablespoons olive oil in a cast iron skillet over medium heat until shimmering.
Sear the chicken breasts for about 4 minutes on each side until brown.
Transfer the cast iron skillet to the oven and let the chicken cook for another 15-20 minutes until fully cooked.
Meanwhile, mix balsamic vinegar and brown sugar in a sauce pan and bring to simmer. Reduce balsamic vinegar until you have a semi-thick sauce.
Once the chicken is fully cooked, drizzle with sauce and serve warm.