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 200 C / Gas 6.
Place some basil leaves, some 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 pan over medium heat. Add stuffed chicken breasts to pan and sear on all sides, 3 to 4 minutes per side.
Transfer pan to the oven and bake until chicken cooked through in the centre and the juices run clear, about 10 minutes.
Allow stuffed chicken breasts to rest for 5 minutes. Drizzle with balsamic reduction before serving.