How to Make It
Place the potatoes in a large pot and cover with cold water. Bring to a boil and add 2 1/2 tablespoons salt. Reduce heat and simmer until just tender, about 15 minutes. Drain the potatoes and return to the pot.
Meanwhile, peel and thinly slice 5 medium shallots. Heat 1 tablespoon olive oil in a medium skillet over medium heat. Add the shallots and cook, stirring occasionally, until soft and golden brown, about 15 minutes. Set aside.
Add the butter, buttermilk, cream, nutmeg, 1/2 teaspoon salt, and 3/4 teaspoon pepper to the cooked potatoes. Mash the potatoes with a handheld masher. Fold in the shallots. Transfer to a serving dish.