Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper and set aside.
Place the chicken breasts on a cutting board. Use a sharp knife to slice each breast horizontally, creating a pocket (don’t cut all the way through). Open the breasts like a book.
Evenly distribute the apple slices inside the chicken breasts. Top with the Brie slices. Fold the chicken back over the filling, pressing gently to secure the stuffed breasts.
Season both sides of the stuffed chicken with salt, pepper, and thyme.
Heat the olive oil in a large skillet over medium-high heat. Cook the chicken in batches (2 breasts at a time) for about 1-2 minutes on each side until golden brown. Transfer the seared chicken to the prepared baking sheet.
In a small bowl, whisk together the maple syrup, creamy Dijon mustard, and whole grain Dijon mustard. Use a pastry brush to coat the tops of the chicken with the glaze.
Place the chicken in the preheated oven and bake for 20-25 minutes or until fully cooked through.