01 - In a medium saucepan, add diced strawberries, granulated sugar, and lemon juice. Stir thoroughly to ensure the strawberries are evenly coated.
02 - Set the saucepan over medium heat. Cook for 5 to 7 minutes, stirring occasionally, until the strawberries soften and begin to release juices.
03 - In a small mixing bowl, whisk together cornstarch and water until fully dissolved and smooth.
04 - Gradually pour the cornstarch slurry into the simmering strawberry mixture. Stir constantly and continue to cook for 3 to 5 minutes, until the filling thickens to a glossy consistency.
05 - Remove the saucepan from heat. Blend in vanilla extract if desired. Allow the filling to cool completely; it will further thicken as it cools.
06 - Transfer cooled filling to an airtight container and refrigerate for up to 5 days.