4 tablespoons butter, divided
1 yellow onion, diced
1 tablespoon minced garlic
1 bag spinach, coarsely chopped
3 tablespoons all-purpose flour
3 cups milk
8 ounces Gruyère, shredded (2 cups)
1 teaspoon salt
1/2 teaspoon freshly ground pepper
4 cups hot cooked brown or wild rice
3 cups cooked chicken breast meat, shredded
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 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.
Melt remaining 3 tablespoons butter in a large pot 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 casserole dish.
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.