Strain and rinse lentils; add to medium pan with water. Simmer with lid tilted for 25-30 minutes, or until tender. Discard any extra liquid. Stir in 1-2 tablespoons of taco seasoning.
While lentils are cooking, peel and dice sweet potato; place on baking sheet. Drizzle with olive oil and 1-2 tablespoons of taco seasoning. Bake at 400 for 20-25 minutes or until caramelized and tender.
Strain and warm up black beans (on stove or in microwave).
For avocado tahini sauce, place all ingredients in a blender and blend until smooth.
On each tortilla pile lentils, black beans, and sweet potatoes. Sprinkle with feta cheese (optional). Drizzle avocado tahini sauce on top.