Dave's Mexican Veggie Tacos

Name: Dave's Mexican Veggie Tacos
Prep time: 20 min
Cook time: 30 min
Total time: 50 min
Cuisine: American, Mexican, Latin American
Ingredients: 1 tablespoon oil, or as needed; 4 carrots, chopped; 1 potato, chopped; 2 leeks, chopped; 1 onion, chopped; 1 sweet potato, shredded; 1 (15 ounce) can black beans, rinsed and drained; 2 tablespoons ground cumin; 1 teaspoon dried oregano; 1 teaspoon chili powder; 1 pinch cayenne pepper; salt and ground black pepper to taste; 16 (6 inch) corn tortillas; 1 avocados - peeled, pitted, and mashed; 0.5 cup crumbled queso fresco; 0.5 cup salsa; 0.25 cup finely chopped fresh cilantro; 0.5 teaspoon lime juice, or to taste