Cream together the ricotta and cream cheese. Add sugar and continue beating until smooth. Add lemon juice, vanilla, flour, and cornstarch, beating well after each addition. Beat in eggs, one at a time. Gradually beat in melted butter. Blend in sour cream, continue beating all ingredients until well blended.
Pour into ungreased 9 x 13 inch pan. Bake at 320 degrees F (160 degrees C) for 1 hr. Turn off oven and leave inside for 2 hours.
Cut into squares. Top each with strawberry, blueberry, or cherry pie filling. Note: Bars can be frozen.