Using a mandolin, a fine slicing disc in a food processor, or a very sharp knife, cut the potatoes into wafer-thin slices. Cook in a large saucepan of lightly salted boiling water for 1–2 minutes until just tender, then drain. Preheat the oven to 220 degrees C.
Heat the oil in the pan and lightly fry the onions with the pancetta for 2–3 minutes until softened, then add the mushrooms and cook for a further 2 minutes. Add the potatoes and rosemary, season with freshly ground black pepper and gently toss, without breaking up the slices.
Place the pizza bases on two lightly greased baking trays, then spread the potato mixture evenly over the tops. Trickle the milk over the topping of both pizzas. Bake for 15 minutes until the potatoes are tender and golden.
Remove the pizzas from the oven, cut them into quarters and place two quarters on each plate. Equally divide the chicken among the pieces of pizza, then scatter the rocket and parmesan shavings over the top. Serve at once.