Preheat the oven to 350 degrees F (175 degrees C).

Combine flour, baking powder, salt, and 1/2 cup sugar in a large bowl. Add oil, egg yolks, water, and lemon zest; beat with an electric mixer until smooth.

Beat egg whites and cream of tartar in a separate clean bowl until peaks form. Add 3/4 cup sugar gradually, and beat until very stiff and shiny peaks form. Fold 1/3 of the egg whites into the batter, then quickly fold in remaining whites until no streaks remain. Turn everything into ungreased 10-inch tube pan.

Bake in the preheated oven until a toothpick inserted in the center comes out clean, about 1 hour. Invert cake and cool completely in pan. When cool, loosen edges and shake pan to remove cake.

To make the filling: Beat cream in a chilled glass or metal bowl with an electric mixer until soft peaks form. Fold in lemon filling; chill filling until stiff.

To assemble the 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, if using.