Preheat your oven to 375°F (190°C).
In a skillet over medium heat, cook the Italian sausage until browned and crumbly. Drain any excess grease and set aside to cool.
Butterfly the chicken breasts by slicing each horizontally, being careful not to cut all the way through, and open them up like a book.
In a bowl, mix the cooled sausage with olive tapenade and grated Parmesan cheese to create the stuffing.
Divide the stuffing mixture evenly among the chicken breasts, placing it on one side of the opened chicken breast before folding the other side over to enclose the filling.
Secure each chicken breast with toothpicks to prevent the stuffing from spilling out during cooking.
In a small bowl, combine olive oil, garlic powder, dried oregano, salt, and pepper.
Brush the olive oil mixture over the stuffed chicken breasts, ensuring each is well-coated.
Place the chicken breasts on a baking sheet lined with parchment paper.
Bake in the preheated oven for 35 minutes, or until the chicken is cooked through and the juices run clear.
Let the chicken rest for 5 minutes before removing the toothpicks.
Garnish with fresh basil leaves if desired and serve.