Peel and finely chop the onion. Peel garlic and chop finely. Heat the oil in a large pot. Fry onion and garlic for about 5 minutes. Deglaze with the broth, add the lentils and bring to the boil.
Stir in tomato paste. Cover the lentils and cook over medium heat for about 45 minutes. In the meantime, clean the celery, put some green aside for garnishing, wash and cut diagonally into pieces. Peel and wash the potatoes and cut them into pieces.
After 25 minutes cooking time add celery and potatoes. Cut the sausages into pieces. Add the sausages to the stew about 5 minutes before the end of the cooking time. Stir in lemon juice. Season to taste with salt, pepper and sugar.
Garnish lentil stew with remaining celery green and serve.