Preheat oven to 375 degrees F.
Cut chicken breasts lengthwise into 6 smaller pieces that you can butterfly.
Slice each piece lengthwise in the middle but not through, open flat and tenderize a bit.
Preheat large non-stick skillet on low-medium heat and swirl half of oil to coat.
Add garlic and onion, sauté until translucent, stirring occasionally.
Add spinach, cranberries, thyme, salt, and pepper; sauté for another minute.
Remove from heat.
Divide spinach mixture into 6 piles.
On each flat open chicken tender, place a slice of brie and 1/6th of spinach stuffing next to it.
Fold in half, tuck in the ends if possible, and wrap tightly with kitchen twine.
Repeat with remaining chicken, place on a large plate, and sprinkle with salt and pepper.
Return skillet to medium heat and swirl remaining oil.
Add stuffed chicken and sear for about 3 minutes per side.
If skillet is ovenproof, cover it or transfer chicken to a baking dish and cover.
Bake for 25 minutes.
Remove from oven, let chicken stand 5 minutes, and remove twine.
Serve hot with a side of choice, spooning juices on top.
