Preheat the Oven - Set the oven rack to the lower-middle position. Preheat to 375ºF (191ºC).
Make the Filling - In a large bowl, combine spinach, artichoke hearts, cream cheese, mozzarella cheese, parmesan cheese, sour cream, garlic, ½ teaspoon salt, and ¼ teaspoon black pepper. Set aside, or cover and refrigerate if not using immediately.
Prepare the Chicken - On the thickest side of the chicken breast, cut a 4 to 5-inch long slit. Continue to cut a 2 to 3-inch deep pocket, leaving about a ½-inch uncut area to keep the other side connected. Repeat with the remaining pieces.
Season the Chicken - In a small bowl, combine 1 teaspoon salt and ½ teaspoon black pepper. Evenly season each side of the chicken with the mixture.
Stuff the Chicken - Add the filling into the pocket of each chicken breast, about ½ cup, do not overfill. If available, close the opening with toothpicks to secure the filling inside.
Sear the Surface - Heat a large oven-proof skillet over medium heat. Once the pan is hot, add the olive oil. Add the chicken, presentation-side down, and lightly press to make contact with the pan. Cook until golden-brown on the surface, about 5 minutes. Flip and cook for 3 minutes. If using smaller breasts, check for doneness, as baking may not be necessary.
Transfer to the Oven - Bake until the meat is no longer pink, and the internal temperature reaches 160 to 165ºF (71 to 74ºC) on a meat thermometer, about 10 to 15 minutes, depending on the thickness.
To Serve - Transfer to a serving dish and rest for 5 minutes. Remove the toothpicks and serve while still hot. Garnish with chopped parsley.
Notes
Filling Yield: Makes about 2 cups.
Storing: Cool completely and store in an airtight container for up to 5 days.
Reheating: Cover and reheat in the microwave on high in 15 to 30-second increments until hot.
Using Frozen Spinach: Defrost and drain 1 cup of frozen chopped spinach.
Make Ahead: The filling can be made separately, 3 days in advance. The chicken can be stuffed, covered, and refrigerated for up to 3 days before cooking.
Bake-Only Directions: Place stuffed chicken on a greased foil-lined sheet pan. Bake at 425ºF (218ºC) for about 20 to 25 minutes, depending on the thickness.