Rinse and drain the dry lentils thoroughly. Combine with the water and bring to a boil. Reduce the heat to low and simmer until tender, about 30 minutes. Drain any excess water.

In a food processor, pulse the cauliflower into rice-size pieces. Heat the olive oil in a large skillet over medium heat. Cook the onion and jalapeño until the onion is translucent, about 5 to 7 minutes. Add the cauliflower, garlic, and half the spices and cook for 4 minutes. Stir in the tomato sauce and cook for another 3 to 4 minutes, or until the cauliflower is tender.

Add the cooked lentils, the remaining half of the spices, and the salt and pepper. Cook for an additional 3 minutes to blend the flavors. Scoop the finished mixture into the taco shells, then top with shredded romaine lettuce and a dollop of guacamole. Serve immediately.