Preheat the oven to 375°F (190°C). Cut the chicken breasts lengthwise into 6 smaller pieces that can be butterflied. Slice each piece lengthwise in the middle without cutting through. Open flat and tenderize slightly. Set aside.

Heat a large non-stick skillet over low-medium heat and add half of the oil. Sauté the garlic and onion until translucent, stirring occasionally. This usually takes about 3 minutes. Add spinach, cranberries, thyme, salt, and pepper; sauté for another minute. Remove from heat.

Divide the spinach mixture into 6 portions. Place a slice of brie and 1/6th of the spinach stuffing in the middle of each flat open chicken tender. Fold in half, tuck in the ends if possible, and wrap tightly with kitchen twine. Repeat with the remaining chicken, place on a large plate, and sprinkle with salt and pepper.

Return the skillet to medium heat, add the remaining oil, and sear the stuffed chicken for about 3 minutes per side until golden brown.

If your skillet is ovenproof, cover it; otherwise, transfer the chicken to a baking dish (and cover). Bake for 25 minutes. Remove from the oven, let the chicken stand for 5 minutes, and remove the twine.

Serve hot with a side of your choice, spooning the juices on top. For a crispy exterior, you can sear the chicken for an additional minute on each side after baking.