• Kosher salt
• 2 1/2 pounds Yukon gold potatoes , unpeeled, cut into 1-inch pieces
• 1 large celery root (1 1/2 pounds), peeled, cut into 1/2-inch pieces
• 5 cloves garlic , peeled
• 1 large shallot , finely chopped
• 2 1/2 cups plain low-fat yogurt , plus 1/2 cup extra if desired
• Freshly 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.