In a bowl, cream shortening and sugar. Beat in egg yolks and milk. Combine flour and baking powder; add to creamed mixture and mix well. Spread into a greased 13x9-in. baking pan. Bake at 350° for 15 minutes or until a toothpick inserted in the center comes out clean.
In a saucepan, combine sugar and cornstarch; stir in water until smooth. bring to a boil over medium heat; cook and stir for 1 minute. Remove from the heat. Stir a small amount into egg yolks; return all to pan. Cook over medium heat for 2 minutes. Remove from the heat. Add butter, lemon juice and zest; set aside.
In a small bowl, beat the egg whites until soft peaks form. Gradually add sugar, beating until stiff peaks form. Beat in cinnamon. Fold in almonds. Spread hot filling over crust. Spread topping over filling. Bake at 350° for 15 minutes or until lightly browned. Cool on a wire rack. Store in the refrigerator.