Preheat oven to 325°F. Grate 1 tsp. lemon peel; cut lemon crosswise in half. Squeeze both halves to measure out 3 Tbsp. juice. Set aside. Beat margarine, sugar and lemon peel in large bowl with electric mixer on medium speed until light and fluffy. Blend in egg yolks. Add lemon juice and flour; mix well. Gradually add milk, mixing after each addition until well blended. Stir in cereal. (Mixture will look curdled, but this will not affect finished product.)
Beat egg whites in medium bowl with electric mixer on high speed 5 to 8 min. or until stiff peaks form. Gently stir into lemon mixture. Pour into greased 1-qt. baking dish. Place dish in pan of hot water.
Bake 1 hour or until top is golden brown and pudding begins to pull away from side of dish. (Pudding will have cake-like layer on top with custard layer below.) Serve warm or cold. Top each serving with 2 Tbsp. of the whipped topping just before serving.