Preheat oven to 325. In a large saucepan, combine sugar and cornstarch. Stir in eggs and lemon juice. Cook over medium heat, stirring constantly with a wire whisk, until mixture comes to a boil. Boil for 3 minutes, stirring constantly. Remove from heat; stir in butter and lemon zest. Pour egg mixture into prepared pie crust. Using the back of a spoon, spread meringue over hot filling, sealing to edge of the pie crust. Bake for 23 minutes. Cool completely on a wire rack. Store in refrigerator.