Chop the strawberries finely, then strain the juice.
In a small bowl, combine the water and cornstarch. Using a spoon, stir until the cornstarch is completely dissolved and a slurry forms. Put aside.
Mix thoroughly with the sugar, lemon juice, and strawberries in a large skillet. Put the strawberries on a medium heat and turn on the gas. Bring the mixture to a boil after 4–5 minutes of cooking.
Using one hand, slowly add the slurry while using the other to mix the filling continuously. It will begin to thicken the strawberry filling. After a minute of cooking, turn off the heat. It should have a thick consistency rather than being watery.
The filling should then be moved to a bowl, covered, and refrigerated to cool fully. When it has cooled fully, it will become slightly thicker.
The filling should then be moved to a bowl, covered, and refrigerated to cool fully. When it has cooled fully, it will become somewhat thicker.
Keep your strawberry cake filling in the refrigerator for up to a week in an airtight container.
Enjoy!