In a medium saucepan, combine the chopped strawberries, granulated sugar, and lemon juice. Stir gently to combine.
Cook the mixture over medium heat, stirring occasionally until the strawberries start to release their juices, about 5 minutes.
In a small bowl, mix the cornstarch with the water until dissolved. Add this mixture to the saucepan.
Continue to cook the mixture, stirring constantly, until it thickens and begins to bubble, about 2-3 minutes.
Remove from heat, and let the filling cool slightly. For a smoother texture, you can blend it with a blender or food processor, or pass it through a fine mesh sieve to remove any large pieces.
Allow the filling to cool completely before using it as a cake filling or topping.
Store any leftover filling in an airtight container in the refrigerator for up to 3 days.