Add the oil to a large frying pan and bring the heat to medium. Add in the garlic and cook for 2 minutes. Add the walnuts and cook 3 more minutes.
Now add the chipotle peppers, lentils, fire roasted tomatoes, cumin, oregano, and salt. Stir and lower heat to simmer, cover, and cook for 15 minutes.
Preheat oven to 300 F. Chop the tomato, avocado, romaine, and red onion for the toppings. Warm up the shells in the oven. Layer in the lentil walnut mixture and follow up with toppings.