Begin by rinsing the cooked lentils under cold water.
In a large bowl, combine the lentils and green peas.
Add the chopped red onion to the mixture.
In a separate small bowl, whisk together olive oil and lemon juice.
Pour the dressing over the lentil mixture and toss well.
Fold in crumbled feta cheese gently to avoid breaking it too much.
Season with salt and pepper according to your taste preference.
Add chopped parsley and mix until evenly distributed.
Taste and adjust seasoning if necessary.
Serve chilled or at room temperature for best flavor.