Heat the olive oil in a large saucepan over medium heat. Once the oil is hot and shimmering, add half of diced red onion and sauté for 5 minutes until soft. Add the minced garlic, tomato paste, taco seasoning and dried lentils. Sauté for another 30 seconds, then pour in the mushroom or vegetable broth. Allow the mixture to come to a boil over medium heat. Boil, uncovered for 10 minutes. Cover the pot, turn the heat down to medium-low, and simmer for an additional 5 minutes or until the lentils are tender.
While the lentils are cooking, bring 200 ml of water to a boil over high heat in a small saucepan. Add the bulgur to the pot and turn the heat down to low. Simmer, covered, for 12-15 minutes or until tender. Transfer the cooked bulgur into the cooked lentil mixture; stir well to combine.
Scoop the avocado halves into a medium bowl and mash with a fork. Add the remaining cup diced red onion, the halved cherry tomatoes and lime juice. Mix well; taste, and add salt as needed.
Build the tacos by dividing the lentil mixture among the taco shells. Top with guacamole and serve.