01 - Place strawberries, granulated sugar, and lemon juice in a medium saucepan. Stir to evenly coat the fruit.
02 - Set saucepan over medium heat; cook for 5 to 7 minutes, stirring occasionally, until strawberries release juices and begin to soften.
03 - Whisk cornstarch and water together in a small bowl until smooth.
04 - Pour the cornstarch mixture into the saucepan. Stir constantly and cook for an additional 3 to 5 minutes, until the filling thickens and achieves a glossy appearance.
05 - Remove from heat and incorporate vanilla extract, if desired.
06 - Allow filling to cool completely before using. It will continue to thicken as it cools. Transfer any leftovers to an airtight container and refrigerate for up to five days.