Preheat oven to highest setting, about 500.
Place a lightly greased, foil-lined baking tray in the oven.
With a sharp knife, score the skin of the pork loin in a criss-cross pattern. Turn it around so that the flesh side is facing upwards and cut a slit along the side of the loin, without cutting all the way through, to open it out like a butterfly. Cover the flesh with the lemon zest, sage and parsley leaves, then scatter over the garlic. Season generously with salt and pepper and drizzle over with a little olive oil.
Roll up the loin and secure tightly with kitchen string in 3-4cm intervals. Rub the scored skin with a large pinch of salt, drizzle with a little olive oil and sprinkle with another pinch of salt. Carefully place the pork on the hot baking tray and roast for 15-20 minutes until the skin is golden and starting to crispen.
Turn down the oven to 350 and roast for approximately 25 minutes per pound until the pork is cooked through and tender. Rest for 10-15 minutes before carving.