Heat oven to 190C/fan 170C/gas 5. Slice both tenderloins along the entire length of the fillet, making sure not to slice all the way through, then open out flat. Place between pieces of cling film and flatten to 5mm thickness by beating with a rolling pin or meat mallet.
Sprinkle the inside of both fillets with the herbs, garlic and half of the oil and season. Roll up tightly, secure with toothpicks and season the outside.
Heat remaining oil in a heavy roasting tin over a medium heat, then brown the loins on each side, turning frequently, for about 5 mins. Transfer to the oven and roast for 15-20 mins. Cover loosely with foil for 5 mins, then slice.
Meanwhile, coarsely grate the apple and toss with the lemon juice and seasoning. Serve the sliced pork with the apple and a green salad.