Pile the cauliflower onto a lined baking sheet and sprinkle with the oil, lime juice, cumin, cayenne, garlic powder, and salt. Toss well to coat and spread out on the sheet. Roast on 400ºF for about 45 minutes, or until the edges are slightly charred.
Meanwhile, chop the onion in small bits and sauté over medium-low heat. Add the black beans and a sprinkle of salt and stir well to marry the flavors.
Warm the tortillas with your preferred method (stovetop, oven, microwave). I usually just toast each one on both sides in a dry skillet. Place 4 tortillas on each of 4 plates and add the black beans, the cauliflower, and your toppings of choice to each tortilla. Enjoy these tacos while they're hot and fresh!
Notes
Optional garnishes: queso fresco, cotija, or feta cheese, salsa or sriracha, avocado, cilantro, more lime juice, etc.