For the tacos:
1 tablespoon olive oil
1/2 large onion, diced
1 1/2 teaspoons chili powder
1/2 teaspoon ground cumin
1/4 teaspoon kosher salt, plus more as needed
1 (15-ounce) can black beans, drained and rinsed
1/4 cup water
8 corn tortillas
1 bag cabbage slaw or shredded cabbage
1 medium avocado, sliced
Heat the oil in a large skillet over medium-high heat until shimmering. Add the onion and cook, stirring occasionally, until softened, about 2 minutes. Stir in the chili powder, cumin, and 1/4 teaspoon salt. Add the beans and water.
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. Taste and adjust seasoning as needed.
Meanwhile, heat the tortillas. Stack them on a microwave-safe plate and cover with a damp paper towel. Microwave in 30-second bursts until warm.
Fill the tortillas with the black bean mixture and top with slaw or cabbage, avocado, and salsa. Serve with lime wedges.