Fruit Pie Filling

Basic pie filling:
Put fruit, sugar, salt, cornstarch, and lemon juice in a saucepan and cook on a low simmer until desired consistency, stirring frequently. Keep a measuring cup of water nearby and add a small amount of water as needed.

For cake filling, such as for Black Forest cake:
Double the recipe.
Mash as the fruit softens and cook until you have a syrupy consistency that will coat a spoon with chunks of fruit in it.
If your fruit is seedy like blackberries, you will want to strain it over a bowl after it breaks down, but before it becomes syrupy. Throw away the solids that have collected in the strainer once you've gotten as much as you can through, and return the liquid to your pan to cook until done.

Filling for Figgy Pudding Cake:
Mash as the fruit softens and cook until you have a syrupy consistency that will coat a spoon with chunks of fruit in it. Strain the mixture and drain away as much excess liquid as you can. Puree the solids to use.