Whisk together first 3 ingredients. Pour evenly into crusts.
Bake at 350° for 15 minutes. Remove to a wire rack to cool. Cover and chill 4 hours.
Beat whipping cream at high speed with an electric mixer until foamy; gradually add sugar, beating until soft peaks form. Spread evenly over chilled pies. Top with fresh lemon slices and a mint sprig before serving, if desired.