Cut the sweet potato into ½ to ¾ inch thick sticks. Arrange on a baking sheet lined with parchment paper. Bake at 375° for 20-30 minutes or until fork tender. Check frequently to avoid burning.
Place the onion, garlic, red peppers and mushrooms & 1-2 tablespoons of water in a large skillet. Cook over low heat until the onion is translucent and the peppers and mushrooms are soft, adding 1-2 tablespoons of water as needed to prevent sticking.
Add the sweet potato sticks, beans, cumin, chili powder, salt & pepper to taste. Gently fold to coat the sweet potato with the spices. Cook over medium low heat until heated through, 5 to 7 minutes. Remove from the heat.
Place the avocado in a small bowl and use a fork to gently mash it.
Warm the tortillas, if using, one at a time over a gas flame or in a toaster oven.
To assemble the tacos, spread some avocado on half of each tortilla or lettuce leaf. Spoon the bean, sweet potato and vegetable mixture on top, then add the tomato, scallions and cilantro. Drizzle with some lime juice. Fold each tortilla in half. Enjoy!