Butterfly the fillets to an even thickness. Place each between 2 sheets of plastic wrap/baking paper and batten out to 3-5mm thickness. Spread each fillet evenly with the Philly. Top with the proscuitto, rind and herbs. Roll to enclose the filling and secure with toothpicks.
Heat the butter in an ovenproof pan and brown the chicken quickly on all sides. Pour in the sherry, cover loosely with foil and transfer to the oven.
Bake the chicken in a moderate oven 180 °C for 20 minutes or until cooked through. Remove the toothpicks and slice each thickly.
Spoon mash onto service plates, top with the chicken and drizzle with remaining sauce in pan. Serve with steamed broccolini.