Preheat oven to 425 degrees F.
Pat chicken dry. Place a chicken breast in a large zip-top bag and zip the top (make sure to release any air in the bag first), then place it on your poultry cutting board. Using a meat mallet like this one, pound to flatten the chicken. Remove from the zip-top bag and reuse bag and mallet to repeat the process with the remaining chicken breast pieces.
Season chicken with salt and pepper on both sides and place in a large mixing bowl or dish. Add spices, minced garlic, extra virgin olive oil, and lemon juice. Combine to make sure the chicken is evenly coated with the spices and garlic.
In a large lightly oiled baking dish or pan, spread onion slices on the bottom. Arrange seasoned chicken on top, and add the tomatoes.
Cover the baking dish tightly with foil and bake for 10 minutes covered, then uncover and bake another 8 to 10 minutes or so. Watch carefully. This can take less or more time depending on the thickness of your chicken breasts. To be sure chicken is cooked through, use an instant digital Cooking or Meat Thermometer for the proper temperature of 165 degrees F.)
Remove from heat. Let chicken breasts rest (cover with foil or another pan) for 5 to 10 minutes or so before serving. Uncover and garnish with fresh parsley and basil. Enjoy!