• 6 tablespoons (¾ stick) butter
• 1 cup finely chopped onions
• 6 tablespoons all-purpose flour
• 1 teaspoon salt
• ½ teaspoon freshly ground black pepper
• 2½ cups milk
• ½ pound fresh mushrooms, sliced
• 5 tablespoons dry sherry
• 5 cups chopped cooked turkey breast
• 2 cups chopped cooked ham
• Two 5-ounce cans water chestnuts, drained and sliced
• 1 cup shredded Swiss cheese
• 2 cups fresh bread crumbs
• 6 tablespoons (¾ stick) butter, melted

Preheat the oven to 400°F. Grease a large, shallow baking dish and set aside.

In a large, heavy skillet, melt 4 tablespoons of the butter over moderate heat, add the onions, and cook, stirring, till soft but not browned, about 3 minutes. Sprinkle the flour, salt, and pepper over the onions and stir about 2 minutes longer. Reduce the heat to low and gradually add the milk, stirring constantly till the mixture is thickened and smooth. Remove the pan from the heat.

In a small skillet, melt the remaining 2 tablespoons butter over moderate heat, add the mushrooms, and cook, stirring, till golden, about 5 minutes. Add the mushrooms to the milk mixture, then add the sherry, turkey, ham, and water chestnuts and stir till well blended. Spoon the mixture into the prepared baking dish and sprinkle the cheese over the top. In a small mixing bowl, combine the bread crumbs and melted butter, spoon the mixture evenly over the cheese, and bake till the casserole is lightly browned, about 35 minutes. Serve hot.