Preheat the oven to 400F. Half or quarter the carrots (depending on how wide they are), then cut them into 1-inch long pieces. Place them in a large mixing bowl with 2 tablespoons olive oil, 1 teaspoon cumin, the coriander, smoked paprika, and the harissa. Toss well to combine. Transfer the carrots to a foil or parchment lined baking sheet and sprinkle with coarse salt. Roast for 30-40 minutes, stirring every 10 minutes, or until the carrots are very tender and a little crispy.
While the carrots roast, heat the remaining 2 teaspoons olive oil in a large skillet. Add the red onion. Sauté for 5 minutes, or until the onion is soft and clear. Add the lentils, the remaining 1/2 teaspoon cumin, the chili powder, lime juice, and salt. Stir to combine everything and heat the lentils up (about 1-2 minutes). Turn off the heat.
Toast the tortillas gently over an open oven burner (about 1 minute per side). Alternately, you can wrap them in foil and place them in a 350F degree oven for about 5 minutes prior to taco assembly.
To assemble tacos, place 1/4 cup lentil mixture, a handful of roasted carrots, and a few avocado slices in each taco. Top with an extra squeeze of lime juice, if desired. Fresh, chopped parsley makes a nice accompaniment, too. Enjoy.