Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add lentils, cover, and steam, adding more water as needed, until tender, 30 to 35 minutes. Transfer cooked lentils to a serving bowl.
Blend oil, vinegar, garlic, oregano, red pepper flakes, and salt in a blender until well incorporated. Add parsley and cilantro; pulse only long enough to incorporate the herbs.
Drizzle 3 tablespoons chimichurri sauce over lentils; add feta cheese to salad. Toss to evenly coat. Store remaining sauce in an airtight container in the refrigerator for another use.