1. Heat 1 tablespoon butter in a large nonstick skillet over medium-high heat. Add onion, and cook, stirring often, until softened. Stir in garlic, and cook 1 minute. Add mushrooms, and cook 8 minutes or until softened. Add half of spinach, and cook, stirring constantly, until spinach begins to wilt. Add remaining spinach, and cook, stirring constantly, 2 minutes or until all spinach has wilted.
2. Melt remaining 3 tablespoons butter in a large Dutch oven over medium-high heat. Add flour, and cook, whisking constantly, 1 minute. Slowly whisk in milk, and cook, whisking constantly, about 5 minutes or until mixture comes to a boil and thickens. Remove from heat, and slowly whisk in Gruyère, whisking until smooth. Whisk in salt and pepper. Add spinach mixture, rice, and chicken; transfer to a lightly greased 13- x 9-inch (4-quart) casserole dish.
3. Cover and refrigerate overnight, or bake, covered with foil, at 375° for 45 minutes or until bubbly and hot. Let stand 5 minutes before serving.