Place the strawberries in a medium saucepan and set over medium-high heat. If frozen, begin cooking until they start to thaw and juices come out.
Add the sugar, cornstarch, and salt and stir to combine. Add the lemon juice and water.
Continue to cook the sauce until the juices are bubbling. Reduce the heat and simmer until the fruit partially breaks down and the sauce thickens, about 5 minutes.
Remove the strawberry cake filling from the stove and transfer it into a separate container to cool. A shallow dish will help it cool and thicken faster.
Store in the refrigerator until ready to use.
Notes
If making in advance, store the strawberry cake filling in a lidded container in the refrigerator for up to 2 weeks.Allow the filling to completely cool and thicken before using it in a cake.Fresh or frozen strawberries are fine.