Preheat oven to 220c/400f/Gas 6. Place the tomatoes in a small roasting tray with 1 tablespoon oil. Season with salt and pepper and roast for 18-20 minutes until tender and slightly charred. Place the potatoes in a pan of boiling, salted water, cover and cook for 10-12 minutes until tender. Remove the leaves from the basil and half of the parsley, and put into a small food processor or blender with 1 tablespoon capers.
Crush in the garlic with the zest of half the lemon and 1 tablespoon olive oil. Process for 4-5 seconds or until the herbs are roughly chopped and the paste is combined. Season with pepper. Using a sharp knife cut a horizontal pocket, lengthwise, in each of the chicken breasts and open it out. Season the flesh well, spread over the herby paste and sandwich the pieces together again.
Freezing the Parma ham makes it easier to handle, so if doing this, remove from the freezer now. Lay the ham out flat, two pieces together and place the chicken on top. Bring the ends of the ham over the top of the chicken to wrap and enclose it.
Heat the remaining oil in a medium frying pan, place the chicken best-side down (ie the side with no seams) and cook for 2-3 minutes each side until the ham is crisp. Put on a roasting tray and cook for 8-10 minutes. Place the pan back on the heat and add 50g/2oz butter. When the butter is melted and starts to sizzle, remove the pan from the heat.
Whisk in the juice of half the lemon to emulsify and thicken the sauce. Add the remaining capers, season with pepper and reserve. Drain the potatoes and mash. Beat in the milk until creamy and keep warm. Heat the remaining butter in a wok. Add the spinach and stir-fry for 1-2 minutes until just wilted, then fold through the mash. Season well with salt and pepper. Gently reheat the sauce. Roughly chop the remaining parsley and stir into the sauce.
Remove the tomatoes from the oven. To serve, remove the chicken from the oven and cut into five slices on the diagonal. Spoon the mash onto the centre of a serving plate, arrange the chicken on top and place the tomatoes on the side. Spoon around the lemon butter sauce and garnish with a sprig of parsley.