Cut a deep pocket into the thick side of each breast. Season inside and out with salt and pepper.
Chop up the tomatoes and 2 slices of the cheese fairly finely. Add the onion and garlic powder along with the italian seasoning, Let it sit for 10 minutes or so.
Divide the tomato mixture between the two breast pockets. Do not overfill, the breasts should still close. secure with a toothpick if necessary.
Beat the egg in a shallow bowl.
Pour the panko crumbs into another shallow bowl.
Dip each breast in the egg, coating well. Place each in the bowl of panko crumbs and coat well, pressing the crumbs into each breast.
Place the breast in a shallow baking dish that has been coated with the cooking spray. Coat the tops of the breast well with the cooking spray.
Bake in a 400 degree oven for 25 minutes or until cooked through.
Top with the remaining 2 slices of cheese and broil for 3 -5 minutes until cheese is melted and bubbly. Allow to cool for 5 minutes and serve.