Place the rinsed lentils in a medium saucepan with the water or broth. Bring to a boil, then reduce the heat and simmer uncovered for 15 to 20 minutes. You want them to be tender but still hold their shape. Drain any excess liquid and let them cool slightly.
While the lentils are cooking, chop the cherry tomatoes, cucumber, and red onion. Place them into a large mixing bowl along with the sliced Kalamata olives and fresh parsley. Having everything prepped before the lentils finish helps the process move quickly.
In a small bowl or a mason jar, whisk together the extra virgin olive oil, fresh lemon juice, minced garlic, dried oregano, salt, and black pepper. Whisk until the dressing is well combined and slightly emulsified.
Add the cooked, slightly cooled lentils to the bowl with the vegetables. Pour the dressing over the mixture and toss gently to coat everything evenly. The warmth from the lentils will help them absorb the flavors of the dressing.
Gently fold in the crumbled feta cheese. Be careful not to over-mix, as you want to keep the feta in distinct chunks rather than turning the salad cloudy. Serve immediately or refrigerate for later.