Step 1

Arrange a rack in the middle of the oven and heat to 425°F. Place a large sheet of aluminum foil on a work surface. Stack the tortillas on top and wrap completely in the foil; set aside.

Step 2

Place the sweet potatoes on a rimmed baking sheet. Drizzle with 1 tablespoon of oil and sprinkle with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Toss to combine and spread into a single layer. Roast for 20 minutes. Flip the potatoes over with a flat spatula and push aside a little to leave one corner of the baking sheet empty. Place the foil packet of tortillas in the empty space and continue to roast until the sweet potatoes are browned and tender and the tortillas are warm, about 10 minutes more. Meanwhile, cook the beans.

Step 3

Heat the remaining 1 tablespoon oil in a large skillet over medium-high heat until shimmering. Add the onion and cook, stirring occasionally, until softened and translucent, about 3 minutes. Stir in the chili powder, cumin, and remaining 1/2 teaspoon salt. Add the beans and water.

Step 4

Cover the pan and reduce the heat to maintain a simmer. Cook for 5 minutes, then uncover and use the back of a fork to partially mash the beans, leaving about half whole. If there's any remaining water in the pan, simmer the mixture uncovered until evaporated, about 30 seconds more.

Step 5

Add the roasted sweet potatoes and cilantro to the black beans and gently toss to combine. Fill the tortillas with the black bean mixture and top with guacamole and cheese, if using. Serve with lime wedges.