In a medium pot, bring 5 cups of water to a boil. Rinse the lentils in a colander. Add the lentils to the pot and bring back to a boil. Continue to cook for 20-25 minutes or until tender. Add more water if necessary. Drain and set aside.
In a large saucepan, heat 1 tablespoon of olive oil over medium heat. Add half a red onion (save the other half for the pico de gallo) and saute for about 5 minutes or until soft. Add the garlic and cook for another minute.
Add the lentils. Add 1/4 cup water, tomato paste, oregano, crushed red pepper, salt, and black pepper to taste.
Meanwhile, make the yogurt sauce. In a small bowl combine yogurt, dill, salt, and garlic. Store in the fridge until you are ready to eat.
Make the Cucumber Pico de Gallo. In a medium bowl combine the tomatoes, onion, jalapeno, cucumber, salt and lemon juice. You can either add cilantro to the Pico or garnish each taco with it.
Heat the corn tortillas by spraying each side with nonstick spray and spreading them out on a baking sheet. Broil for about 2-3 minutes, then flip each tortilla and broil again for 2 minutes. Alternatively, you could heat them on the stovetop.
To build the tacos, spoon some lentils down the center of a tortilla, top with yogurt sauce, Cucumber Pico de Gallo, and feta.