Preheat oven to 375.
Mix chicken, rice, soup, mayonnaise, waterchestnuts, onion, and lemon juice in 9x13" dish. (Optional: Add 3 chopped hard-boiled eggs.) Bake 30 minutes. Meanwhile, melt margarine in saucepan. Stir in cornflake crumbs and almonds.
Spread topping on casserole. Bake another 15 minutes.