4 tablespoons butter, divided
1 medium onion, halved vertically and thinly sliced crosswise (about 1 cup)
2 garlic cloves, minced
1 pound shiitake or other mushrooms, stems discarded and caps sliced
2 (12-ounce) bunches spinach, tough stems removed and coarsely chopped OR 2 (5-ounce) packages prewashed 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 (12 ounces)

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.