Line a large baking sheet with parchment paper and set aside. Preheat the oven to 375°F. Lay the chicken breasts on a cutting board and slice each one horizontally without cutting all the way through, so it opens like a book.
Lay the chicken open on the prepared baking sheet. Season the inside with garlic salt and pepper. Layer apple slices across one side of each breast, then top with 3 to 4 slices of Brie. Repeat with the remaining chicken.
Fold the empty side of each breast over the apples and Brie. Bake for 25 minutes. While it bakes, make the glaze.
In a large pan, combine the maple syrup, apple juice, garlic, thyme, and Dijon mustard. Stir over medium-high heat and let it simmer for 10 to 15 minutes until it's thick enough to coat the back of a spoon.
After 25 minutes, brush the glaze over the chicken breasts. Remove pan with chicken from the oven and preheat to high broil.
While the oven preheats, add foil to a fresh baking sheet (or remove parchment and replace with foil on the one you just used). Place the chicken breasts on the newly prepared pan.
Once to temp, slide the pan with breasts into the oven approximately 5–6 inches below a fully heated High Broiler, and broil for 1–3 minutes, watching carefully. Chicken is fully cooked at 165F internal temp.
Plate over mashed potatoes, or next to the side of your choice, then garnish with fresh parsley if you like, and serve.