Preparation

• Position a rack in the center of the oven, and heat the oven to 400°F. Grease a 3-qt. gratin dish with 1 Tbs. of the butter.
• 在一个中等大小的锅,小火加热油。一个dd the leeks and cook, stirring often, until softened, about 3 minutes. Set aside.
• Melt the remaining 4 Tbs. butter in a heavy-bottomed medium saucepan over low heat. Add the flour and cook gently, stirring, for 2 minutes. Gradually add the milk, whisking well between each addition to break up any lumps. Once all the milk has been incorporated, increase the heat to medium high, and cook, whisking continuously, until the sauce has thickened, 4 to 5 minutes. Reduce the heat to low, and using a wooden spoon, stir in the Cheddar until melted. Season with 1/4 tsp. salt, 1/4 tsp. pepper, and the nutmeg. Stir in the mustard. Remove from the heat, and then cover.
• Fill a medium bowl with cold water, and add the lemon juice.
• Bring a large pot of well-salted water to a boil. Peel and quarter the celery root. Cut into 1/4-inch-thick slices, placing them in the lemon water to prevent them from discoloring.
• 一个dd the celery root to the boiling water and cook for 4 minutes. Remove from the pot with a slotted spoon to a large rimmed baking sheet lined with paper towels. Add the parsnips and carrots to the pot, cook for 3 minutes, then transfer to the towel-lined baking sheet. Put the celery root, parsnips, and carrots in a large bowl, and toss together.
• Sprinkle half of the Gruyère evenly over the bottom of the prepared gratin dish. Top with half of the vegetables. Spoon over slightly less than half of the cheese sauce. Season with salt and pepper, and top with the leeks, spreading them in an even layer. Add the rest of the vegetables in an even layer, gently pressing down to create even layers, and pour over the remaining cheese sauce. Sprinkle evenly with the remaining Gruyère. Sprinkle with the Parmigiano and then the panko. Place the gratin dish on a large rimmed baking sheet, and bake until light golden, 30 to 40 minutes. Let rest for at least 20 minutes, and then slice and serve.