Preheat oven to 375°F and line a baking sheet with parchment paper
Place chicken breasts on a cutting board and slice horizontally without cutting all the way through so each piece opens like a book
Lay chicken open on the baking sheet and season the inside with garlic salt and pepper
Layer apple slices on one side of each chicken breast and top with Brie slices
Fold the empty side of the chicken over the filling to close and bake for 25 minutes
While chicken bakes make the glaze by combining maple syrup, apple juice, garlic, thyme, and Dijon mustard in a saucepan over medium-high heat
Let the glaze bubble and thicken for 10 to 15 minutes then reduce heat to low and stir occasionally
After 25 minutes remove chicken from the oven and brush the glaze generously over each breast
Broil for 10 minutes or until the tops are golden brown and caramelized
Serve warm with mashed potatoes and garnish with parsley if desired