1. Preheat oven to 375 degrees F. Cut chicken breasts lengthwise to create 6 pieces and butterfly each piece by slicing horizontally without cutting through completely. Open the chicken flat and tenderize lightly.
2. Heat half the olive oil in a non-stick skillet over low-medium heat. Add minced garlic and onion, sauté until translucent. Add spinach, cranberries, thyme, salt, and pepper, cooking until spinach wilts. Remove from heat.
3. Divide the spinach mixture into 6 portions. Place one slice of brie and one portion of stuffing in the center of each butterflied chicken piece. Fold the chicken over the filling, tucking in ends if possible, and secure with kitchen twine or toothpicks. Season outside of chicken with salt and pepper.
4. Heat remaining olive oil in the skillet over medium heat and sear the stuffed chicken for about 3 minutes per side.
5. Transfer chicken to an ovenproof dish, cover, and bake for 25 minutes. Remove from oven and let rest for 5 minutes. Remove twine or toothpicks before serving.
6. Serve hot with pan juices spooned over the chicken. Accompany with a side of choice like mashed potatoes, rice, or vegetables.