Bring a large pot of water to a boil and cook lentils according to package directions or until tender, approximately 15-17 minutes. Once cooked, strain and rinse under cold water to cool, and set aside.
While the lentils are cooking, prepare the vegetables and dressing. Chop the tomatoes, cucumber and red onion into bite-size pieces, and mince the parsley.
In a small bowl or jar, combine the olive oil, lemon juice, mustard, garlic, sea salt and black pepper. Set aside.
Once the lentils have cooled, combine lentils and vegetables in a large bowl and stir to combine.
Cover with dressing and toss well to combine and ensure that the salad is well coated with the dressing.
Sprinkle crumled feta cheese, toss again, and season with additional salt and pepper to taste.
The salad can be serve immediately or stored in the fridge for up to 3 days.