2. Add the lentils, the stock, and the rosemary and bring to a boil. Reduce heat, cover, and simmer gently for 45-50 minutes, or until the lentils are very soft. If it is too thick at this point (I like it this way, but you might not), you can add more liquid and cook your soup a bit more. Add the vinegar, mix well, and season to taste with coarse sea salt before serving.