01 - Place diced strawberries, granulated sugar, and lemon juice in a medium saucepan. Stir until strawberries are evenly coated.
02 - Set saucepan over medium heat and cook, stirring occasionally, for 5–7 minutes until strawberries begin to soften and release juices.
03 - In a small mixing bowl, whisk together cornstarch and water until fully dissolved.
04 - Pour cornstarch slurry into saucepan and stir continuously. Cook for an additional 3–5 minutes until the filling becomes thick and glossy.
05 - Remove pan from heat and stir in vanilla extract, if using.
06 - Allow filling to cool completely before using. Transfer to an airtight container and refrigerate for up to 5 days.