Preheat oven to 325 degrees. Grease a 2-1/2 quart casserole dish with butter.
Bring a large pot of lightly salted water to a boil. Cook pasta, according to package instructions, stirring occasionally, until cooked al dente. Drain.
Melt 2 tablespoons of butter in a large skillet over medium heat; sauté celery until it begins to soften, about 2-3 minutes. Stir in mushrooms; cook and stir until mushrooms are softened, about 3 minutes. Stir in minced garlic and cook for 1 minute more. Remove vegetables from the skillet.
In the same pan, melt the remaining 2 tablespoons butter, whisk flour into hot butter to make a paste. Slowly whisk in warm chicken broth and bring to a simmer; reduce heat to low and cook, whisking constantly, until sauce is thickened. Stir in cream, 1/2 cup parmesan cheese, salt, and white pepper.
Add turkey and sautéed vegetables to the sauce; mix in pasta. Transfer mixture to prepared casserole dish. Sprinkle with more parmesan cheese.
In a small skillet, melt 1 tablespoon of butter; stir in panko bread crumbs until crumbs are coated. Sprinkle bread crumbs over parmesan cheese.
Bake for 30 minutes, until the sauce is bubbling and crumb topping has browned.
Cook's Note: We used smoked turkey in this recipe and it was AMAZING!