01 - In a medium saucepan, mix together the diced strawberries, granulated sugar, and lemon juice until the fruit is evenly coated.
02 - Place the saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until strawberries release juices and begin to soften.
03 - In a separate small bowl, whisk together cornstarch and water until fully dissolved and smooth.
04 - Pour the cornstarch mixture into the saucepan. Stir constantly while cooking for an additional 3 to 5 minutes, until the mixture thickens and turns glossy.
05 - Remove the saucepan from heat. Stir in vanilla extract if using.
06 - Allow the fruit filling to cool completely. The mixture will continue to thicken as it cools.
07 - Transfer any leftover filling to an airtight container and store in the refrigerator for up to 5 days.