Preheat the oven to 400°F. Butter the bottom and sides of a 1.5-quart casserole dish. Put the cream, rosemary, garlic, lemon zest, 2 teaspoons salt and some black pepper in a large saucepan. Peel and slice the potatoes 1/8 inch thick and add to the pan. Bring to a boil over medium heat and cook, stirring occasionally so the potatoes do not stick and are completely submerged, until the sauce thickens, about 5 minutes. Pour the mixture into the prepared baking dish and sprinkle with the Parmesan. Put the dish on a baking sheet and bake until the potatoes are tender and the top is brown and bubbling, about 40 minutes. Let stand 10 minutes before serving.