If you are making homemade pesto, toast the pine nuts in a small (20cm or 8") frying pan.
Transfer them to a mini blender or pestle and mortar, add the remaining ingredients and blitz until you are happy with the texture. I usually begin a pesto in a mini blender, then transfer it to a pestle and mortar to finish it off.
Slice the mozzarella cheese into 1cm (just under ½") thick slices.
Butterfly the chicken breasts, taking care not to cut all the way through and flatten a little with the side of your fist.
Season the inside of the chicken with a little sprinkle of salt and pepper, take half of the pesto and divide it between the two chicken breasts, spreading it over the chicken.
Add the sliced mozzarella and close up the chicken; you might want to pin the chicken together with a couple of toothpicks. This is not essential, but it makes the dish look better when it is finished.
Drizzle over a little oil, add a sprinkle of coarse sea salt to the skin, place the chicken on a roasting tray and bake for 23-28 minutes at 200°C or 390°F. Ensure that the chicken reaches 73°C or 165°F before serving.
Spoon the remaining pesto over the chicken when you serve the dish.