Finely chop the onions, carrots and celery. In a large heavy based saucepan melt the duck fat and add the onions, carrots and celery, cook until the vegetables have softened. Add the lentils and stir well. Add the stock, the chopped potato and bouquet garni. Bring to the boil, reduce heat and simmer for approximately 45 minutes or until the lentils are soft and nearly all the stock has been absorbed.