Preheat oven to 350°F.
Stir together chicken, rice, and soup in a large bowl. Set aside.
Heat 2 tablespoons of the butter in a medium skillet over medium-high. Add celery and mushrooms, and cook, stirring often, until tender, about 5 minutes. Add to chicken mixture.
Stir in mayonnaise, chicken broth, lemon juice, and grated onion. Spoon mixture into an 11- x 7-inch baking dish.
Microwave remaining 2 tablespoons butter in a microwave-safe bowl on HIGH until melted, about 20 seconds. Toss together cornflakes, almonds, and melted butter in a medium bowl. Sprinkle over chicken mixture.
Bake in preheated oven until golden brown and bubbly, about 40 minutes. Serve immediately.
