Add the chopped strawberries, lemon juice, and caster sugar to a medium saucepan set over medium heat.
Heat until the sugar has melted and the strawberries have softened.
Mix the water and cornflour together in a small bowl and add this cornstarch slurry to the saucepan.
Allow the water from the fruit to evaporate and for the filling to thicken. Once the filling coats the back of a spoon, remove the saucepan from the heat and transfer the strawberry mixture into a glass jar to cool completely.
Notes
Watch the heat of the stovetop, if it gets too hot the strawberry mixture will catch on the bottom of the saucepan.
Allow the strawberry mixture to cool properly before using it as a cake filling.