Spice mix: Combine chilli powder, cumin, coriander, oregano, salt and fennel seeds in a small bowl.
Taco filling: Heat oil in a large pan over medium-high heat. Cook onion and garlic, stirring occasionally, until lightly browned, about 3 minutes. Add spice mixture and cook, stirring, until toasted, about 30 seconds.
Reduce heat to medium and add lentils, tomato puree, a few splashes of water and chipotle peppers. Cook, mashing lightly with a fork and adding water if necessary, until lentils are heated through and hold together, about 5 minutes. Season with additional salt if needed and chilli sauce.
Lightly toast tortillas in a pan over medium heat. Spread taco filling down centre of each tortilla. Top with lettuce, tomatoes and fresh coriander. Serve with guacamole and lime wedges.