In a medium saucepan, combine the diced strawberries with the sugar, lemon juice and lemon zest. Heat over medium-low heat and bring to a simmer.
2 cups strawberries, 4 tablespoons white sugar, 1.5 tablespoons lemon juice, zest of 1 lemon
Simmer for 25-30 minutes, stirring frequently. You may need to lower the heat to low heat if it is bubbling too much.
You'll know the strawberry sauce is ready when you can coat the back of a spoon with the sauce and run a line through it with your fingers.
Take the pan off the heat and let cool completely. Store in a large mason jar or airtight container in the fridge for up to 1 week.
Notes
I recommend using a weighing scale to weigh out the strawberries, using a scale instead of measuring cups is a more accurate way to bake!More tart? If you want the strawberry filling to be less sweet, use just 3 tablespoons of sugar instead of 4.Smoother filling: if you want the filling to have less chunks of strawberries, make sure you dice up the strawberries into small pieces. It'll be easier for them to break down in the saucepan.How much strawberry cake filling does this make? This makes enough to fill a 2 layer 8-inch cake.