Heat 1 tablespoon of olive oil in a skillet over medium heat. Add the chopped onion and cook until soft, about 5 minutes.
Add the minced garlic, chili powder, and cumin to the skillet. Cook for 1 minute until fragrant.
Add the rinsed black beans and water to the skillet. Bring to a simmer. Use a fork or potato masher to lightly mash about half of the beans to create a thicker texture.
Stir in 1/4 teaspoon of salt. Cook for 3-5 minutes until the liquid reduces slightly. Keep warm.
Prepare the slaw: In a medium bowl, combine the shredded cabbage and chopped cilantro.
In a small bowl, whisk together the lime juice, 1 tablespoon of olive oil, and a pinch of salt.
Pour the dressing over the cabbage and cilantro mixture. Toss well to coat.
Warm the tortillas according to package directions (usually a quick warm in a dry skillet or microwave).
Assemble the tacos: Fill each warm tortilla with the black bean mixture, top generously with the cilantro lime slaw, and add any desired optional toppings.
Notes
For extra flavor in the bean filling, add 1/2 teaspoon of smoked paprika.
If you prefer a softer slaw, let it sit for 10 minutes before serving to allow the lime juice to slightly soften the cabbage.
You can use canned or pre-shredded cabbage to save prep time.