Grate peel from lemon (about 4 to 5 teaspoons); set aside. Squeeze juice from lemon, adding water if necessary to equal 1/4 cup; place in 2-quart saucepan. Stir in sugar and butter. Heat over medium heat until butter is melted. Remove from heat; stir in beaten eggs. Cook over low heat, stirring constantly, until mixture thickens. Stir in lemon peel. Cool slightly, about 15 minutes. (Lemon filling can be refrigerated at this point until ready to serve).