Place lemon juice and milk in a cup. Mix flour, sugar, baking powder, baking soda and salt into a large mixing bowl. Add shortening, banana and sour milk mixture; mix until all flour is dampened. Beat vigorously for 2 minutes. Add eggs; beat 2 minutes longer. Stir in walnuts. Bake in 2 greased and floured (I use Pam/ flour spray.) 9 inch round pans in 350 degree oven for 35 minutes. Cool 10 minutes in pans then remove from pan. Frost with sour cream frosting.