Cook the lentils in boiling water for about 8-10 minutes. Cut the beetroot, mushrooms and cucumber into slices and the spring onions into diamonds. Drain the finished lentils in a sieve, stir in the spring onions and season with salt and pepper.
Mix the vinegar, mustard, salt, pepper and sugar together. Withhold oil. Serve the beetroot, cucumber and mushrooms with the lukewarm lentil mixture and drizzle with the mustard dressing.