Preparation

• Put the potatoes in a large saucepan and cover with cold water by at least an inch. Add a generous 1/2 tsp. salt and bring to a boil. Lower the heat to maintain a steady simmer, cover the pot partially, and cook until the potatoes are quite tender when tested with a metal skewer, 15 to 20 minutes.
• Meanwhile, heat the crème fraîche (or heavy cream and sour cream) in a small saucepan over low heat, stirring occasionally, until smooth and just hot. Set aside in a warm spot.
• Drain the potatoes and dump them back into the pan. Dry the potatoes over medium heat, shaking the pan and stirring until the potatoes look floury and leave a light film on the bottom of the pan. If using a ricer, dump the potatoes into a bowl and then rice them back into the pot set over very low heat. If using a hand masher, mash them in the pot until completely smooth.
• Using a wooden spoon, beat in the lemon zest and butter. Add the crème fraîche in small additions, about 1/4 cup at a time, beating well after each addition. Season with salt and pepper and serve right away.