Prepare the ingredients and set the oven rack to the lower-middle position. Preheat to 375ºF (191ºC).
In a large bowl, combine softened cream cheese, parmesan cheese, spinach, sun-dried tomato, basil, parsley, chives, garlic, ½ teaspoon salt, and ¼ teaspoon black pepper. Set aside, or cover and refrigerate if not stuffing chicken right away.
On the thickest side of the chicken breast, cut about a 4 to 5-inch long slit. Continue to cut a 2 to 3-inch deep pocket, leaving about a 1-inch uncut area to keep the other side connected. Make sure not to cut through the bottom. Repeat with the remaining pieces.
In a small bowl, combine 1 teaspoon salt and ½ teaspoon black pepper. Evenly season each side of the chicken with the mixture.
Evenly spread the cheese mixture into the pocket of each chicken breast, about 2 to 4 tablespoons, do not overfill. If available, close the opening with toothpicks to secure the filling inside.
Heat a large oven-proof skillet or Dutch oven over medium heat. Once the pan is hot, add the olive oil. Add the chicken, presentation-side down, lightly press the meat to make contact with the pan. Cook until golden brown on the surface, about 5 to 7 minutes. Flip over and cook for 5 minutes. If using smaller chicken breasts, check for doneness as baking may not be needed.
Transfer the pan to the oven. Bake until the meat is no longer pink and the internal temperature reaches 160 to 165ºF (71 to 74ºC), about 10 to 15 minutes, depending on the thickness of the meat. Rest the chicken for 5 minutes, then serve hot.
Notes
Herb substitution: Use 1 tablespoon dried Italian seasoning for fresh basil, parsley, and chives.