1) Preheat the oven to 180°C / gas mark 4.
2) Line two heavy, large baking sheets with silicone baking sheets or parchment paper. In a food processor, chop the Parmesan, Gruyere and rosemary together until coarsely chopped. Set the cheese mixture aside.
3) Separate the dough strips. Using a pizza cutter or a large sharp knife, cut each dough strip in half lengthwise to form thin strips. Working with one dough strip at a time, coat each strip with the cheese mixture, pressing very gently. Twist each cheese covered dough strip and place onto the prepared baking sheets. Sprinkle with the salt, if you wish.
4) Put the trays into the oven and bake until the breadsticks are golden brown, about 10 to 15 minutes.
Transfer the warm breadsticks to a basket and serve.