Use a paring knife to remove tough fibers from the outside of the rhubarb stem (do not discard the peels). Cut the stem crosswise into medium slices.
Peel the apples (do not throw away the peels) and cut into medium cubes. Remove the mint leaves from the stems.
Put the apple and rhubarb peels and chopped mint stems into a saucepan, pour 1.2 liters of cold drinking water, add salt and 3 tbsp. l. Sahara. Bring to a boil and cook for 15 minutes.
Strain the broth, put rhubarb and apples in it, bring to a boil again. Add more sugar if desired. Simmer until the rhubarb is soft, about 15 minutes.
Remove the saucepan from the heat, blend the soup with an immersion blender until about half the pieces remain, return to the fire.
Mix starch with 2 tbsp. l. cold water and pour into boiling soup. Mix thoroughly for 1.5-2 minutes. Serve soup hot or cold with cream. Chop mint and add for garnish.