Add the lentils and water to a large pot and bring to a boil.
Once boiling, lower the heat to a simmer and cook for 15-20 minutes until tender but not mushy.
Drain and rinse under cold water so they stop cooking and cool fast. Add them to a large bowl.
Chop the cucumber, red onion, red pepper and parsley and add them to the bowl with the lentils.
Whisk all dressing ingredients in a medium bowl. Alternatively, add all ingredients to a mason jar and shake until well combined.
Pour the dressing over the lentils and vegetables and stir everything together.
Stir in the feta, if using, and serve over a bed of arugula or baby spinach, if desired. You can also simply mix the greens into the salad.
