Makes 12 servings
*Ingredients available at the farmers’ market
- 2 pounds celeriac (celery root)*, peeled and cut into 1-inch pieces
- 3 pounds potatoes*, peeled and cut into 1-inch pieces
- 1 cup whole-milk plain Greek yogurt*
- 4 tablespoons unsalted butter*
- 1 tablespoon Dijon mustard
- 1¼ teaspoons salt
- 1 teaspoon ground pepper
- ¼ cup chopped fresh green onions*, plus more for garnish
- 1½ cups thinly sliced shallots*
- 6 tablespoons extra-virgin olive oil
- Place celeriac in a large pot and add 3 inches of water. Cover and bring to a boil over high heat. Add potatoes, cover and return to a boil. Reduce heat to a simmer and cook until the vegetables are very tender about 25 minutes.
- Meanwhile, preheat oven to 400°F. Coat a 3-quart shallow baking dish with cooking spray.
- Drain the vegetables and return to the pot. Mash with a potato masher to a coarse consistency. Add yogurt, butter, mustard, salt, and pepper; mash until mostly smooth. Add green onions and stir to combine. Spoon the mixture into the prepared baking dish. Smooth the top. Tightly cover the pan with foil.
- Bake for 45 minutes. Uncover and continue baking until the top is browned, about 30 minutes more.
- Meanwhile, prepare fried shallots: Separate shallot slices into rings. Heat oil in a small skillet or saucepan over medium-high heat until shimmering. (To test if it’s hot enough, add a shallot ring: if it starts sizzling on contact, the oil is ready.) Add half the shallots and cook, stirring frequently, until browned, 4 to 6 minutes. Remove pan from heat and use a slotted spoon to transfer the shallots to a paper-towel-lined plate. Cook the remaining shallots and transfer to the plate. They will get crispier as they cool. (Discard remaining oil.)
- Let the casserole cool for 10 minutes. Serve topped with the fried shallots and more chives, if desired.