Preheat the oven to 400º F.
Cut chicken horizontally, but do not cut all the way through and open them up. Spread interior of each chicken with 2 tablespoons of pesto each. Then evenly divide the chopped spinach over the pesto. Fold the chicken together to close it.
Heat an oven proof skillet over medium high heat. Add 1 tablespoon of olive oil. Carefully add chicken. Sear on each side for 2-3 minutes. Finish baking in the oven for about 8 minutes or until the meat reaches 165º F.
Take the skillet out of the oven and place tomatoes and cheese. Put the skillet back in the oven until cheese starts to bubble, about 2-3 minutes.
For the dipping sauce, combine yogurt and pesto in a small bowl. Serve over the chicken or on the side.