Preheat the oven to 180 degrees C. Line a baking dish with baking paper or foil.
Take the chicken breasts and smother them in the pesto on all sides, adjust pesto quantity to your taste. Season the chicken with salt and pepper, to taste.
Place the chicken breasts in the baking dish and bake in the preheated oven for 30 minutes. At 30 minutes quickly take the chicken breasts out of the oven and sprinkle with mozzarella then with diced tomato. Place the chicken back in the oven and cook a further 20 to 25 minutes until the chicken is no longer pink in the centre and the cheese is melted.