For the mushroom saute: Bring a large pot of salted water to a boil.
Melt the butter in a large saute pan set over medium heat. Add the thyme and shallots, and saute until the shallots begin to brown, 1 minute. Add the mushrooms and 1/2 teaspoon salt. Saute for 5 minutes, stirring often. Add the sherry and cook until it evaporates and then add several grinds of black pepper.
Cook the spaghetti to al dente. Drain well and cool.
For the sauce: Preheat the oven to 350 degrees F. Grease a casserole dish with nonstick cooking spray.
Melt the butter in a saucepan set over low heat. Add the flour, stirring constantly for 5 minutes to make a roux. Add the half-and-half, 1 cup at a time, stirring constantly. Increase the heat to medium and cook, stirring often, until the sauce thickens, 5 to 8 minutes. Season with salt and pepper.
In a large mixing bowl combine the mushroom saute, cooked pasta, chicken and white sauce with 2 1/2 cups of the Cheddar. Season with salt and pepper. Add to the prepared casserole dish and top with remaining 1/2 cup cheese, the breadcrumbs, the remaining 1/2 cup melted butter and a dusting of smoked paprika. Bake for 20 to 25 minutes. Then, cover the casserole with foil and cook for 10 more minutes. Serve hot or cool to room temperature to store in the refrigerator or freezer for later use.
This casserole can be frozen for up to 3 months. For the shredded chicken, you can use a roasted or rotisserie chicken, skin removed and chopped.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.