Wash the plums and put them in a pot along with the frozen dark sweet cherries.
Add cinnamon stick and the cloves.
Cover the fruits with water and place the pot over medium high heat.
Bring to a boil.
Reduce the heat to medium low and let cook for 30 minutes.
Strain the fruits through a strainer.
Make sure to press on the cooked fruits to get the maximum pulp.
You can add water on the fruits in the strainer to help get more pulp.
Pour the strained liquid back in the pot over medium heat.
Add granulated sugar (more or less your liking).
Add lemon juice and lemon slices.
If you like you can leave the juice concentrated and add cold water later at serving but I prefer to add water at this step.
Mix everything with a spoon or a whisk.
In meantime, continue pressing on the cooked fruits in the strainer to get more pulp.
Add the pulp to the pot.
Bring to a boil and let cook for about 5 minutes. Make sure you taste your drink (if it needs more sugar then add more. If it needs more water then add more).
Remove from heat and let cool completely.
Strain again.
Serve it cold with ice if you like. You can always add more cold water at serving.