01 - Heat oven to 400°F and line a rimmed baking tray. Cut a pocket in each chicken breast without cutting through. Divide apple and Brie among pockets, leaving edges clear.
02 - Season with salt, pepper and thyme. Sear briefly in the measured oil, handling carefully so filling stays inside, then transfer to the tray.
03 - Mix maple syrup and both mustards. Brush over chicken and bake, beginning checks around twenty minutes, until thick meat and filling centers reach 165°F.
04 - Rest briefly. If toothpicks were used, count and remove every one before serving.