Place the sugar and vinegar in a small saucepan and heat until the sugar has dissolved. Quarter the apples, remove the cores and finely dice the apples. Add to the vinegar and sugar mixture and leave to cool.

Finely chop the parsley and stir it, together with the oil, into the cooled vinegar and apple mixture. Pour the sauce over the lukewarm lentils, mix briefly and season with salt and pepper.