Mix olive oil, garlic, onion powder, black pepper, and sea salt together in a bowl. Place chicken breasts in a shallow dish and pour marinade on top. Refrigerate at least 4 hours.

Heat balsamic vinegar in a small saucepan to a slow boil. Allow to boil for 15 minutes, watching carefully to prevent burning. Cook until vinegar has reduced to half the original volume. Allow balsamic reduction to cool completely.

Preheat the oven to 400 degrees F (200 degrees C).

Place 1/4 cup basil leaves, 2 ounces mozzarella cheese, and 1/2 a tomato on each piece of chicken. Roll chicken up and secure with toothpicks.

Heat olive oil in an oven-safe skillet over medium heat. Add stuffed chicken breasts to skillet and sear on all sides, 3 to 4 minutes per side.

Transfer skillet to the oven and bake until chicken is no longer pink in the center and the juices run clear, about 10 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).

Allow stuffed chicken breasts to rest 5 minutes. Drizzle with balsamic reduction before serving.