Lemon Filling (Sugar Mixture)
(a) Whisk together the granulated sugar, cornstarch and salt in a medium saucepan.
(b) Add the lemon juice and water and whisk to combine. Cook over medium-high heat, stirring frequently, until mixture comes to a boil.
(c) Cook the sugar mixture around 6 minutes or until it begins to thickening and bubbling. Once thickened, give it a whisk and reduce heat to low.