1/2 cup (1 stick) unsalted butter, divided
1/2 cup finely chopped shallots
1 pound fresh baby spinach, or 1 (16 oz) package frozen spinach, defrosted and squeezed dry
1/4 teaspoon ground nutmeg
salt and freshly ground black pepper to taste
6 boneless chicken breasts
1/4 cup gluten-free flour
1 1/2 cups chicken broth
1 cup heavy cream
1/4 cup cream sherry
1/2 cup finely shredded Swiss cheese
1/2 cup freshly grated Parmesan cheese
Make it now:
Coat a 9-by-13 inch baking dish with nonstick cooking spray.
Melt 3 tablespoons of butter in a large skillet over medium heat. Add shallots and cook, stirring, until softened, about 3 minutes. Add spinach and nutmeg, and cook, stirring, until the spinach is wilted and cooked though, another 3 minutes. Season with salt and pepper, transfer to the prepared baking dish, and let cool.
Wipe out skillet with a paper towel. Melt remaining 5 tablespoons butter over medium heat. Sprinkle chicken with salt and pepper, and saute for 3 minutes on each side, until the pieces begin to turn golden brown. Put chicken in baking dish on top of the spinach.
Whisk flour into remaining butter in the pan and cook over medium-high heat, stirring constantly, until white bubbles form on the surface. Cook mixture for 2 to 3 minutes, until smooth and thick. Slowly add broth, whisking until the sauce comes to a boil. Add cream and sherry. Pour sauce over the chicken in the baking dish.
Allow the casserole to cool. Sprinkle with Swiss and Parmesan cheeses. Cover and refrigerate for up to 2 days or freeze for up to 6 weeks.
Bake it later:
Defrost casserole in the refrigerator, overnight if necessary.
Preheat the oven to 350°F. Allow casserole to come to room temperature, about 30 minutes.
Bake casserole for 35 to 40 minutes, until sauce is bubbling and the cheeses are melted and golden brown. Serve hot.