Chicken and Rice Casserole with Spinach and Shiitakes

We used a mixture of Lundberg Wehani and wild rice, but in this casserole any mixture of brown rice, regular brown rice or wild rice, or barley will be delicious. Leave the chicken for a vegetarian main course. Great for snacks.
Heat 1 tablespoon butter in a large nonstick skillet over medium-high heat. Add the onion and cook, stirring frequently, until softened. Add the garlic and cook for 1 minute. Add the mushrooms and cook for 8 minutes or until softened. Add half of the spinach and cook, stirring constantly, until the spinach begins to wilt. Add the remaining spinach and cook, stirring constantly, for 2 minutes or until all the spinach is cooked through.Melt the remaining 3 tablespoons of butter in a large roasting pan over medium-high heat. Add the flour and cook, whisking constantly, for 1 minute. Slowly pour in the milk and cook, whisking constantly, for about 5 minutes or until the mixture boils and thickens. Remove from the heat and slowly beat the Gruyere, whisking until smooth. Season with salt and pepper. Add the spinach mixture, rice, and chicken; transfer to a lightly oiled 13-inch x 9-inch (4-liter) roasting pan.Cover and refrigerate overnight or bake, covered with foil, at 375 ° C for 45 minutes or until bubbling and hot. Allow to infuse for 5 minutes before serving.