Celeriac adds a bright celery-like flavor and lightens the texture in this healthy mashed potato casserole. The fried shallots are reminiscent of the crispy onions that top a green bean casserole.
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 degrees 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 sour cream (or yogurt), butter, mustard, salt and pepper; mash until mostly smooth. Add chives 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.
To make ahead: Refrigerate unbaked casserole (Steps 1 & 3) for up to 2 days; let stand at room temperature for 1 hour before baking.
2 vegetable, 1 1/2 starch, 2 fat