Wash the lentils, peel the potatoes and carrots and chop them roughly. Peel the onion and chop it roughly too.
Put all ingredients in a saucepan, add the water and bring to the boil, then simmer over medium heat for approx. 25 minutes. The potatoes and carrots should be done. Puree the soup with a hand blender, add the butter and salt and bring to the boil again briefly. Remove the soup from the stove and serve immediately.
The soup becomes a little firmer after cooling, just add a little water the next time it is warmed up. Before enjoying, season to taste with a few drops of lemon and possibly a little pepper.