Place a small saucepan on medium-high heat; add balsamic vinegar and brown sugar; stir until sugar is dissolved. Cook until reduced by half, or until thick enough to coat the back of a spoon, 10-15 minutes. Once done, remove from heat.

Holding a knife parallel to the cutting board, cut the chicken breasts in half horizontally, forming thin fillets.

Mix the oregano, garlic powder, onion powder, salt, and black pepper together in a small bowl. Rub chicken fillets with the spice mixture on both sides.

Heat a grill pan or skillet over medium-high heat. When hot, coat grill pan with olive oil.

Place the chicken fillets in the grill pan; cook until they are golden brown and cooked through, 5-6 minutes per side. Once done, remove from heat.

Preheat oven to broil and position rack under the broiler. Line a baking sheet with foil for easy clean-up (optional).

Combine the tomatoes, shallots, garlic, and julienned basil in a bowl. Add the oil, salt, and pepper and stir to combine. Set aside.

Place cooked chicken on a baking sheet pan. Top with sliced mozzarella. Place in the oven and cook for 2-3 minutes, until the cheese begins to melt.

To serve, divide the chicken between plates, top with tomato mixture, and a drizzle of balsamic glaze.