Set your oven to 375 degrees Fahrenheit and prepare a baking sheet with parchment paper or spray a broil-safe skillet with nonstick spray.
Carefully slice each chicken breast horizontally to create a pocket, making sure not to cut all the way through; season the inside with kosher salt and black pepper.
Place thin slices of honeycrisp apple inside each chicken pocket, then add thin slices of brie cheese on top; fold the chicken back to enclose the filling.
Transfer the stuffed chicken breasts onto your prepared sheet or skillet and bake in the oven for about 25 to 30 minutes until nearly cooked.
While the chicken bakes, combine apple cider or juice, pure maple syrup, minced garlic, dijon mustard, fresh thyme leaves, and kosher salt in a small skillet over medium heat; simmer until the mixture thickens and reduces by half, roughly 12 to 15 minutes.
Switch your oven to broil, brush the thickened maple glaze over the chicken breasts, and broil for 10 to 12 minutes until the chicken reaches an internal temperature of 165 degrees Fahrenheit; watch carefully to avoid burning the glaze.
Serve the chicken warm, drizzling any extra maple glaze over the top for added flavor.