Soak the lentils in cold water at least 24 hours before starting to cook.
Then rinse the lentils in a colander under cold water, put them in a saucepan and fill up with water, well covered. Add a pinch of salt and the parsley and simmer for about 15 minutes over medium heat. Now bring to the boil, when the lentils boil, stir in the sauce thickener. After a minute, remove from heat and season with vinegar and sugar so that it tastes sweet and sour. I always add vinegar and sugar teaspoon by teaspoon so that it is easier to dose. Finally stir in the butter and let it rest for about 1 minute.