Put the potatoes in a large saucepan with the cream and milk. Bring to a boil over medium heat, then add 1 teaspoon of salt. Reduce the heat and simmer until the potatoes are very tender, about 20 to 25 minutes. Remove about 2 cups of the liquid and reserve. Add the lemon zest and season with salt and pepper, to taste. Use a potato masher to break up the potatoes while adding about 1 cup of the reserved liquid, to just 'wet' the potatoes. Add more, for desired creaminess. Spoon the potatoes into a serving bowl and serve immediately.