Preheat oven to 350 F.
Pour cake mix into a large bowl, add the egg and oil; mix until well blended, mixture will be slightly dry. Reserve one cup for the topping, and pat the rest into an ungreased 9 x 13 inch pan.
Bake for 15 min in the preheated oven, set aside to cool, meanwhile, in a medium bowl, beat the cream cheese with the sugar and lemon until smooth.
Spread the mixture evenly over the baked crust and crumble the reserved cake mix on top.
Bake for an additional 15 min or until the filling is set and topping is slightly toasted.
Cool before cutting, store in refrigerator.