1Blend butter and cream cheese together; stir in flour. Cover and chill dough for 1 hour.
2On a lightly floured surface, roll dough to 1/8-inch thickness. Cut in 3-inch rounds with a scalloped cookie cutter. Pat rounds into 1¾-inch muffin pans. Bake tart shells at 400°F (200°C) for 10 to 12 minutes. Cool in pans.
3To make the filling: grate 1/2 teaspoon lemon peel; set aside. In a saucepan, combine the well-beaten eggs, sugar, lemon juice, and butter or margarine. Cook and stir over low heat until thick, about 15 minutes.
4Remove from heat; stir in lemon peel. Chill. Spoon a rounded teaspoonful of filling into each cooled shell. If desired, top with toasted slivered almonds.