Cook the lentils in enough salted water for about 40 minutes until they are cooked but still firm to the bite. Drain and rinse with clean water.
Cut the mushrooms into slices, finely dice the celery. Fry the mushrooms and celery in a pan with a little vegetable oil until the mushroom water has boiled away. Salt and pepper in between. Finally stir in the lentils and heat for 1 minute.
Put the rocket in a large bowl. Add the lentil and mushroom mixture and the dressing, mix everything well and serve immediately.