Preheat oven to 425°F. Toss diced sweet potatoes with olive oil, salt, and pepper. Roast on a baking sheet until edges caramelize and potatoes are tender, about 20 minutes.

While potatoes roast, heat oil in a skillet over medium heat. Cook chopped onion until translucent, add minced garlic, chili powder, and cumin. Stir until fragrant, about 2 minutes.

Transfer roasted sweet potatoes to skillet with aromatics, stir in drained black beans. Cook on medium-low heat until beans are warmed through and flavors meld, about 3–4 minutes.

Wrap corn or flour tortillas in foil, heat in oven alongside filling for last 5 minutes or warm individually in a hot skillet until pliable and lightly browned.

Spoon sweet potato and black bean filling into warm tortillas. Top with fresh cilantro, sliced avocado, queso fresco or shredded cheese, a dollop of sour cream, and lime wedges.

Arrange tacos on a platter, garnish with extra cilantro and lime wedges. Serve immediately while warm for the best taste and texture.