01 - In a medium saucepan, combine diced strawberries, granulated sugar, and lemon juice. Stir to evenly coat the fruit.
02 - Place the saucepan over medium heat and cook for 5 to 7 minutes, stirring occasionally, until the strawberries release their juices and soften.
03 - In a small mixing bowl, whisk cornstarch and water together until completely smooth.
04 - Add the cornstarch mixture to the saucepan, stirring constantly. Continue to cook for 3 to 5 minutes until the mixture thickens and develops a glossy sheen.
05 - Remove the saucepan from heat. Stir in vanilla extract, if using.
06 - Allow the filling to cool fully before using. The mixture will thicken further as it cools.
07 - Transfer any leftover filling to an airtight container and refrigerate for up to 5 days.