In a medium sauce pot combine the cherries, red currants, and 310 ml of the cherry juice. Heat on medium high until boiling.
Meanwhile, whisk together the sugar and starch. Then add in the remaining cherry juice and whisk again.
Once the mixture has come to a boil, turn the heat down to medium and whisk in the starch mixture.
Whisk continuously for about five minutes while the mixture cooks and thickens.
Remove the pot from the heat and pour in the frozen raspberries. Stir to combine.
Pour the contents into a heat safe container and place in the fridge to cool for at least one hour before serving.
Notes
You can use any combination of berries for this recipe - I just recommend using mostly red berries! This will keep best, covered in the fridge for up to 1 week. I love having rote grütze on top of anything from oats to yogurt to ice cream.