In a large bowl, combine flour, baking powder, salt, and 1/2 cup sugar. Add oil, egg yolks, water and lemon rind. Beat with an electric mixer until smooth.
In a small bowl, beat egg whites and cream of tartar until peaks form. Gradually add 3/4 cup sugar, and beat until very stiff and shiny peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain. Turn batter into ungreased 10-inch tube pan.
Bake at 350 degrees F (175 degrees C) for 60 minutes or until a toothpick inserted in the center comes out clean. Invert cake and cool completely in pan. When cool, loosen edges and shake pan to remove cake.
To Make Filling: Beat cream to stiff peaks. Fold in lemon filling. Chill until stiff.
To Assemble Cake: Slice cake horizontally into 3 equal layers. Fill layers with 1/3 cup of filling. Spread remaining filling on top layer. Decorate with lemon slices.
