Cook the lentils in boiling water for about 20 minutes. In the meantime peel and wash the carrots and celery and cut them into small pieces. Clean, wash and cut the leek into rings. Rinse pre-cooked lentils.
Bring the vegetables, lentils and broth to the boil, season with thyme and cook for another 20 minutes. In the meantime cut the fish fillet into portions, wash and pat dry. Season with salt and pepper, turn lightly in flour and tap.
Fry in hot oil while turning for about 8 minutes, season with lemon juice. Season lentil vegetables with salt, pepper, mustard and vinegar. Serve fish and lentil vegetables garnished with lemon and thyme.