1. Preheat the oven to 425 degrees Fahrenheit and line a large, rimmed baking sheet with parchment paper. Toss the prepared sweet potatoes with the olive oil and cayenne pepper (if using). Arrange in a single layer and bake for 30 to 40 minutes, tossing halfway, until the sweet potatoes are tender.
2. Heat olive oil in a large saucepan over medium heat. Add the onions and cook, stirring occasionally, until the onions have softened and are turning translucent, about 5 minutes. 
3. Add the cumin and chili powder and cook for about 30 seconds while stirring. 
4. Pour in the beans and water. Stir, cover and reduce heat to a simmer. Cook for 5 minutes.
5. Add lime juice then mash about half of the beans with fork or potato masher. Season to taste with salt and pepper.
6. Warm the tortillas in a large skillet over medium heat or wrapped in an oven.
7. Assemble the tacos with tortilla, a spread of black beans, sweet potatoes, and any additional toppings you desire. Try the homemade salsa below.

Tomato Salsa

1. Chop the onion and mince the garlic. Place in a bowl. 
2. Add the salt and the lime juice to the garlic and onions, stir, and set aside for a few minutes while chopping the other ingredients.
3. Chop the jalapeño into a fine dice. To reduce heat, remove the seeds and membrane. Add to the bowl.
4. Chop the tomatoes into medium chunks and add to the bowl. 
5. Roughly chop the cilantro and add to the bowl. Stir gently to combine.
6. Adjust seasonings as needed

Cilantro Lime Dressing

1. Whisk all ingredients. Adjust to desired taste.
2. Save in airtight container refrigerated up to 5 days.