Preheat the oven to 400°F (200°C).
Carefully cut a pocket lengthwise into each chicken breast, being careful not to slice completely through.
Season the chicken all over with kosher salt, black pepper, onion powder, garlic powder, and dried oregano. Rub with olive oil.
Heat an oven-safe skillet over medium-high heat. Add the chicken and sear for about 3 minutes per side, until lightly golden. Remove the chicken from the skillet.
Core the apple and slice it thinly. Toss the slices with fresh lemon juice to help prevent browning.
Fill each chicken breast pocket with apple slices and pieces of Brie.
Secure the chicken with toothpicks if needed, then return the stuffed chicken to the oven-safe skillet.
Transfer the skillet to the preheated oven and bake for 15–20 minutes, or until the thickest part of the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven and let the chicken rest for a few minutes before serving.
Garnish with fresh thyme, if desired, and serve warm