Frank McClelland insists that you can tell a cook's skill by his or her potatoes. "You should never boil a potato," he insists. "The cells explode and they become gritty. A perfect potato should just evaporate on the tongue." To achieve this delectable state, he simmers his spuds gently for 15 minutes, then takes them off the heat to finish cooking.
Place whole potatoes in a large (at least 6-quart) stockpot and cover with cold water. Add salt, peppercorns, bay leaves, and garlic. Place over high heat and bring water just to a simmer, not a full boil. Reduce heat to medium and simmer potatoes 15 minutes, checking the water often and adjusting heat as needed.
After 15 minutes, turn off heat and cover pot. Let potatoes sit in hot water until tender when pierced with a sharp knife, 30-40 minutes longer.
Meanwhile, in a small saucepan over medium heat, warm cream and butter until steaming.
Remove potatoes from hot water and transfer to the bowl of a standing mixer or, if using a hand-held mixer, a large bowl. Add hot cream and butter; blend with a whisk attachment until potatoes are smooth. Add buttermilk, mix well, and check consistency. If you want softer potatoes, add more buttermilk, 1/4 cup at a time. Add nutmeg and serve.