1. In a saucepan, combine lentils and water. Bring to a boil, then reduce heat and simmer for 20-25 minutes until lentils are tender. Drain and set aside.
2. In a frying pan, heat olive oil over medium heat. Add chopped onion and cook until translucent, about 5 minutes. Add minced garlic, cumin, and paprika. Cook for another 2 minutes.
3. Add cooked lentils to the frying pan. Stir to combine and cook for another 5 minutes. Season with salt to taste.
4. Warm tortillas in a dry pan or microwave. Fill each tortilla with a portion of the lentil mixture, shredded lettuce, diced tomato, and cucumber. Add a dollop of yogurt if desired.