Preheat oven to 375°F. Cut chicken breasts lengthwise into 6 smaller pieces that you can butterfly. Then slice each piece lengthwise in the middle but not through. Open flat and tenderize a bit. Set aside.
Preheat large non-stick skillet on low-medium heat and swirl half of oil to coat. Add garlic and onion, saute until translucent, stirring occasionally. Add spinach, cranberries, thyme, salt and pepper; saute for another minute. Remove from heat.
Repeat with remaining chicken, place it 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.