Cut deep slits in both sides of each chicken breast, being careful not to cut all the way through the chicken.
In a medium bowl stir together yogurt, herbs, salt, and pepper. Mix chicken into the yogurt mixture, making sure yogurt completely coats each piece and gets into the slits. Allow to marinate for 2-24 hours.
Preheat oven to 350 degrees F. Cut four pieces of foil to about 18 inches long. Place a piece of chicken diagonally in the center of each piece of foil. Bring two corners to the center, and fold down. Fold and pinch other two corners to the middle to close and make a seal. (See image.)
Place foil packets on a cookie sheet, and bake 20-30 minutes. (Very thick cuts of chicken will need the longer bake time.)
Carefully open the foil (watch out for steam!!), and fold down the sides to make a little boat. (You don’t want to lose the juices inside.) Spread about 1/4 cup cheese over each piece of chicken, and return to the oven for about 5 minutes or until the cheese is melted.
To serve, place a bed of spinach leaves on a plate, and carefully pour the juices from a foil packet over the spinach. Then, transfer the hot, cheesy, greek yogurt chicken on top of the spinach leaves. Enjoy immediately.
Serving the chicken over spinach is optional. It would also be great over rice!