Rinse the lentils and put them in a pot. Add plenty of water, salt and boil for about 30 minutes, until soft. Strain them and allow them to cool.
In the meantime we prepare the vegetables: peel the onions, rinse and slice them into rings.
Peel the cucumber and cube it.
Rinse the scallions, strain and chop them.
Then, put the lentils in a salad bowl, add the scallions, onions, cucumber, red peppers, parsley and mix.
When ready to serve, prepare the vinaigrette: put in a bowl the olive oil, vinegar, salt and pepper to taste and stir to combine.
Pour it over the lentil salad, mix and serve.