Wash the potatoes, bring to the boil covered with water and cook for about 20 minutes. Peel and wash the carrot and cut it into sticks. Boil up the stock and cook the carrot sticks for about 8 minutes. Add the lentils 5 minutes before the end of the cooking time.
Drain the potatoes, quench, peel and slice. Squeeze the orange. Mix juice with vinegar. Whip oil into it. Mix potatoes, lentils, carrots and the marinade. Season to taste with salt and pepper.
Clean lamb's lettuce, wash thoroughly and fold into the lentil and potato salad. Arrange salad on plates and serve.