Preheat the oven to 375°F (190°C).
Prepare the stuffing mixture by combining the diced apple, Brie, walnuts, thyme, salt, and pepper in a bowl.
Butterfly each chicken breast by slicing it horizontally but not all the way through, creating a pocket for the filling.
Stuff each chicken breast with the apple and Brie mixture, then use toothpicks to secure them if necessary.
Season the outside of the chicken with salt and pepper, then heat olive oil in an oven-proof skillet over medium-high heat.
Sear the stuffed chicken breasts for about 3-4 minutes per side until golden brown.
Transfer the skillet to the preheated oven and bake for 25-30 minutes until the chicken is fully cooked and the juices run clear.
Let the chicken rest for 5 minutes before slicing to serve.
