Preheat and Prep: Set your oven to 375 degrees Fahrenheit. Pat the chicken breasts dry with paper towels and lay them on a cutting board.
Create the Pocket: Using a sharp knife, carefully cut a horizontal pocket into the thickest part of each chicken breast, being careful not to cut all the way through. The pocket should be large enough to hold about 2 tablespoons of filling.
Make the Filling: In a medium bowl, combine the softened cream cheese, chopped sun-dried tomatoes, fresh spinach, cheddar cheese, garlic powder, salt, and pepper. Stir until well mixed and uniform in color.
Stuff the Chicken: Divide the filling evenly between the two chicken breasts, carefully spooning it into each pocket. Don't overstuff, as some filling may leak during cooking, which is normal.
Season the Outside: Season the exterior of both chicken breasts generously with salt and pepper, and optionally sprinkle with additional garlic powder or Italian seasoning.
Prepare the Pan: Lightly butter or oil a baking dish large enough to hold both chicken breasts without crowding. Place the stuffed breasts in the center.
Bake: Transfer to the preheated oven and bake for 28 to 32 minutes, or until the internal temperature reaches 165 degrees Fahrenheit when measured with a meat thermometer inserted into the thickest part of the chicken (not the filling).
Rest: Remove from the oven and let the chicken rest for 3 to 5 minutes before serving. This allows the juices to redistribute, keeping the meat tender and moist.