Start by rinsing the lentils under cold water in a fine mesh strainer to remove any debris.

In a medium saucepan, combine the rinsed lentils with water and salt. Bring to a boil, then simmer uncovered for 18-22 minutes until tender.

Drain the lentils and spread them out to cool for about 10-15 minutes.

In a bowl, whisk together the olive oil, lemon juice, mustard, garlic, oregano, salt, and pepper.

In a large mixing bowl, combine cooled lentils with cucumber, tomatoes, onion, feta, and parsley. Toss well.

Pour the dressing over the salad and toss gently to combine. Adjust seasoning if needed.

Serve immediately or store in an airtight container for up to 4 days, adding feta just before serving.