In heavy saucepan, dissolve cornstarch in water; stir in sweetened condensed milk and egg yolks. Cook and stir until thickened and bubbly. Remove from heat; add margarine and vanilla; Cool slightly.
Slice 2 bananas; dip in ReaLemon brand lemon juice and drain. Arrange on bottom of prepared crust. Pour filling over bananas; cover. Chill 4 hours or until set.
Spread top with whipped cream. Slice remaining banana; dip in ReaLemon brand, drain and garnish top of pie.
Please Note: Nutritional values listed may vary from final results depending on actual measurements and brands used to prepare the recipe.