Prehat oven to 400. Bring a large pot of salted water to boil for pasta.
In a large sauce pan, melt 2 tablespoons butter over high heat. Add mushrooms and season with salt and peper. Cook, turning frequently, until tender and browned, 8-10 minutes. Add celery and sautee until softened. Transfer to a bowl and set aside.
Make sauce: in same saucepan, melt remaining butter over medium heat. Add flour; cook, whisking, about 1 minute. Whisking constantly, gradually add milk, broth, and sherry. Bring to a boil, reduce to simmer, and add 2 cups parmesan and thyme. Season with salt and pepper.
Cook pasta 2-3 minutes less than package directions; drain. Add to pot and toss well with turkey, mushrooms, and sauce. Divide between two shallow 2-quart baking dishes. Sprinkle with remaining parmesan.
Bake until browned, about 30 minutes, or freeze. Let stand 10 minutes before serving.
To freeze: let pasta cool to room temp, cover tightly with foil, freeze up to 3 months. Bake, covered with foil, from frozen at 400 until center is warm, about 2 hours. Uncover and bake 20 minutes until top is browned.