Blend ketchup and lemon juice; combine with apples. (Note: If apples are very tart, add 1 to 2 tablespoons sugar to ketchup mixture.) For topping, combine flour, sugar and cinnamon; cut in butter until thoroughly mixed. Fill pie shell with apples; sprinkle topping over apples. Bake at 425° 40 to 45 minutes or until apples are cooked. Serve warm with ice cream, if desired.