Combine the lentils and water in a saucepan and bring to a boil. Reduce heat and simmer until all the liquid is absorbed (about 15 minutes). Set lentils aside.
In a small bowl, combine the radish, red onion, lime juice and olive oil. Season with salt, and set aside.
Heat a skillet to medium high heat. Add canola oil and heat for about 1-2 minutes. Add the zucchini, oregano, garlic powder and salt to skillet and cook for 8-10 minutes.
Warm the tortillas in the microwave, on a grill or in a hot oven.
Assemble the tacos. To the tortilla add a scoop of lentils and zucchini and top with radish salsa and Greek yogurt (if you're using).