This traybake is a German classic which I've made gluten free, and I've gussied up fresh cherries in case you can't find sour German cherries in a jar or tin. Sweet 'n sharp; gooey yet firm - this is a very easy cake to make, and tastes utterly delicious.
The original recipe from Luisa Weiss's Classic German Baking says to use sour cherries from a jar or tin (680g), and thicken the jar/tin syrup with cornflour (no water/juice/vinegar required).
Ready-made cherry pie filling would probably work well here. Warm it in a saucepan before spreading on the batter (no cornflour/water/juice/vinegar required). This cake can also be made with frozen blueberries, Italian prune plums, or sugared rhubarb instead of cherries.