Bring the water to a boil in a sufficiently large saucepan. In the meantime, rinse the lentils under running water until the water runs clear. This step is necessary to remove the excess starch and possible dirt.
Peel and clean the potatoes and onion and cut into large pieces. Now add the lentils, potatoes and onions to the boiling water and simmer with the lid closed over medium heat for about 30 minutes.
When the vegetables are soft, puree the soup with a hand blender. It should be mashed until it has a nice, creamy consistency.
Season the soup with the dried mint, chilli flakes and salt.
The soup tastes best when you let it cool down and then reheat it. If the soup is too thick after warming up, just add a dash of water.