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