Add 10 ounces frozen strawberries and ½ cup granulated sugar to a small pot and bring to a simmer over medium-high heat, stirring frequently. Let simmer for 5 minutes.
In a small prep bowl, mix together 1 tablespoon lemon juice and 1 tablespoon cornstarch with a fork.
Once the berries start breaking down, quickly pour in the cornstarch and lemon mixture while stirring.
Continue stirring for 2 minutes, until the mixture has thickened and coats the back of a spoon. Use a potato masher to break down the berries further if needed; you want the filling to have small chunks, not big ones.
Then remove from the heat and let cool in the pot before using in the desired recipe.