Cook the lentils in boiling water for 8-10 minutes. Clean and wash spring onion and cut into rhombs. Clean, clean and slice mushrooms diagonally. Cut beetroot into thin slices.
Wash the cucumber, cut lengthwise into slices. Drain lentils, stir in spring onions and season with salt and pepper. Mix vinegar, mustard and 1 tablespoon of water. Season with salt, pepper and sugar.
Arrange beetroot, cucumber, mushrooms and the lukewarm lentil mixture on a plate and sprinkle with the mustard dressing.