Start by preheating your oven to 375°F so it's ready when you need it. Slice your apple thinly and cut the brie into slices as well, then mince your garlic clove for the glaze. If the brie feels too soft to slice cleanly, pop it in the freezer for about ten minutes first.
Using a sharp paring knife, cut a wide pocket into the side of each chicken breast without slicing all the way through. Think of it like opening a little pouch rather than butterflying the meat completely. Large chicken breasts make this step much easier since they give you more room to work with.
Layer an equal amount of apple slices and brie into each pocket you just created. I usually fit three or four slices of each into every chicken breast, which gives a nice balance of filling without overstuffing. If you're worried about the filling spilling out, a couple of toothpicks can hold things together during searing.
In a small bowl, mix together the kosher salt, onion powder, garlic powder, thyme, and black pepper. Drizzle one tablespoon of olive oil over the stuffed chicken breasts, then rub the seasoning blend all over so every side is coated evenly.
Heat the remaining tablespoon of olive oil in a large oven-safe skillet over medium-high heat. Once it's shimmering, brown the chicken breasts on each side for a couple of minutes per side. You're not trying to cook them through yet, just building color and flavor on the outside.
Transfer the entire skillet into your preheated oven and bake for 25 to 35 minutes. The chicken is done once it reaches an internal temperature of 165°F at the thickest part, not counting the filling. If the tops are browning too fast, loosely tent the pan with foil.
While the chicken bakes, whisk together the maple syrup, Dijon mustard, apple cider vinegar, and minced garlic. Once the chicken comes out of the oven, drizzle the glaze generously over each piece and let it rest for about five minutes before serving.