The mashed potatoes can be made a day in advance. To reheat, place in a large bowl set atop simmering water. Add 3/4 cup milk, heating slowly and stirring occasionally until hot, about 20 minutes.
Servings: Serves 8–10
Ingredients Kosher salt2 1/2 pounds Yukon gold potatoes , unpeeled, cut into 1-inch pieces1 large celery root (1 1/2 pounds), peeled, cut into 1/2-inch pieces5 cloves garlic , peeled1 large shallot , finely chopped2 1/2 cups plain low-fat yogurt , plus 1/2 cup extra if desiredFreshly ground black pepper to taste
Boil 8 cups of water in a large pot with 2 tablespoons salt. Add potatoes, celery root and garlic. Return to boil and reduce heat to medium, cooking uncovered until potatoes and celery root are very tender, 15 to 20 minutes.
Drain potato mixture in a colander. Return to pot; add shallot and yogurt. With back of a fork, smash potatoes against sides of pot until mostly smooth but still slightly chunky. If creamier texture is desired, add additional 1/2 cup yogurt. Add salt and pepper to taste.