Dave's Mexican Veggie Tacos

Heat oil in a skillet over medium heat; cook and stir carrots until lightly browned, about 5 minutes. Add potato; cook and stir until lightly browned and mostly tender, 5 to 10 minutes. Add leeks and onion; cook and stir until onion is lightly browned, 5 to 10 minutes. Mix shredded sweet potato, black beans, cumin, oregano, chili powder, cayenne pepper, salt, and pepper into carrot-potato mixture; cook and stir until sweet potato is tender, about 10 minutes. Remove skillet from heat and transfer sweet potato mixture to a bowl, scraping skillet clean with a rubber spatula. Heat each tortilla in the same skillet over medium heat until warmed, about 30 seconds per side. Spread mashed avocado onto each warmed tortilla and top with sweet potato mixture, queso fresco, salsa, cilantro, and lime juice.