Preheat the oven to 425 degrees . In a large pot, melt the butter over medium-low heat. Add the shallots and cook, stirring occasionally, until they start to soften, about 3 minutes. Stir in the kale, garlic and thyme; season with salt and pepper. Cook, stirring occasionally, until the kale wilts, about 2 minutes. Sprinkle with the flour and cook, stirring, until blended, 2 minutes.
Stir in the stock, cream and nutmeg. Cook over medium heat, stirring occasionally, until the sauce is thickened, 4 to 5 minutes; season. Off heat, stir in the chicken, celery root and carrots; season.
Transfer 1 piecrust to a 9-inch pie pan, allowing the excess dough to hang over the rim. Fill with the chicken mixture; top with the remaining piecrust. Trim the excess crust, then press the edges together to seal. Using the tines of a fork, crimp the edges of the crust. Using a small, sharp knife, cut 5 slits in the center to allow steam to escape. Place the pie pan on a rimmed baking sheet to catch any spillover. Bake until the crust is golden, covering the edges with foil if browning too quickly, 30 to 40 minutes. Let stand 5 minutes before serving.