Well, this one was an accidental discovery, but it's turned out to be a very useful one.
One time, I was trying to make a fruit pie, and for whatever reason, the filling just refused to thicken properly. At a loss, I tried using it as a topping for some pancakes instead, and I was just blown away by how well it all went together. Since then, I've messed around with using it as a topping/filling for several cakes and pies. It's incredibly versatile, allowing plenty of room to be adapted based on what flavor you want or what ingredients are available, and hell, it can even be eaten by itself, if you don't want to go through the hassle of putting it on top of or into something else.
There's often a good 20-30 minutes of waiting around while cooking a cake, so why not use that time to make an extra little something that'll give the cake another dimension of decadent delight, right?
2 granny smith apples (or about 2.5 - 3 cups of any relatively dense fruit, e.g. pears, quince, mangoes, etc.)
1 cup of watery fruit (e.g. cherries, raspberries, plums, etc.)
2 Tbsp maple syrup
1 Tbsp ground cinnamon
1 tsp ground nutmeg
Chop the fruits into bite-sized or smaller pieces.
Grate the lemon's rind into a saucepan.
Add the juice from the lemon.
Add the fruits, cinnamon, nutmeg, and maple syrup. Mix until the fruit pieces are all coated.
Cook covered over medium heat, stirring occasionally, until the pieces of dense fruit have softened.
Remove from heat and allow to cool before using it further.