Preheat the oven to 200C/400F/Gas 6.
Melt 100g/3½oz of the butter in a saucepan, then add 250ml/7fl oz of the stock and stir well to combine.
In a large mixing bowl, mix together the two slices of chopped air-cured ham, cubed bread, pancetta, 30g/1¼oz of the parsley leaves, and all of the shallots and lemon zest until well combined. Season, to taste, with salt and freshly ground black pepper.
Add the dry mixture to the butter and stock mixture and stir well to combine. Set aside to soak for 15-20 minutes.
Meanwhile, using wet fingers, gently pull back the skin of the chicken breasts until all of the skin from the opening to the wing bone comes away from the meat.
Press two pieces of the air-cired ham together and rub them all over with water. Slide two of the ham slices under the skin on one side of the breast, pushing them right to the wing bone.
Rub the sage leaves in water and slide 4-5 leaves under the skin on one side of the breast too. Repeat the process on the other side of the breast.
Squeeze the stuffing mixture dry, then stuff the cavity of the chicken with the stuffing. Place the stuffed chicken into a roasting tray, breast-side down.
Pierce the chicken all over using a wooden skewer, then rub the skin all over with olive oil and season, to taste, with salt and freshly ground black pepper.
Transfer the chicken to the oven and cook, breast-side down, for one hour in the middle of the oven.
Turn the chicken over and continue to roast for a further 20 minutes, or until the skin on the crown is crisp and golden-brown and the chicken is cooked through. (The chicken is cooked through when the juices run clear when pierced in the thickest part with a skewer.) Transfer the chicken to a warm plate, cover with aluminium foil and set aside to rest for 10 minutes (reserve the fat in the roasting tray for the sauce).
Meanwhile, boil the potatoes in a pan of salted water for 18-20 minutes, or until tender. Drain well, then season, to taste, with freshly ground black pepper and stir in the remaining chopped parsley. Keep warm.
Meanwhile, add the remaining 500ml/17fl oz of chicken stock to the fat in the roasting tray and heat over a medium heat until simmering. Continue to simmer for 4-5 minutes, or until the volume of sauce has reduced by half. Keep warm.
Meanwhile, heat the remaining 50g/2oz of butter in a separate saucepan over a medium heat. Add the onions and sweetcorn kernels and stir to coat them in the butter, then cover the pan with a lid and continue to cook for 4-5 minutes, or until softened.
Remove the pan from the heat, stir in the thyme leaves and season, to taste, with salt and freshly ground black pepper. Mash the sweetcorn kernels with a potato masher or the back of a wooden spoon until creamy.
To serve, carve the twice-stuffed chicken and arrange the meat on four serving plates. Spoon the potatoes and creamed sweetcorn alongside. Drizzle over the sauce.