Add the cauliflower, sliced peppers and the sliced onions to a large bowl and add the oil and seasonings. Toss well, ensuring that all of the vegetables are well coated in the spices and the oil. Alternatively, you can place the cauliflower, peppers and onion directly onto 1-2 baking sheets and drizzle with seasonings and oil, then toss well. I use my hands for this, but tongs will work too.
Place the vegetables in a single layer on a large baking sheet, you may need two baking sheets to ensure that the vegetables can be placed in a single layer. Roast for 30-35 minutes, until the vegetables are tender and slightly caramelized, stirring halfway through. I like to check the vegetables about 20 minutes in, stirring the vegetables and tasting, adding more salt as needed or a little more oil if the vegetables seem try.
While the vegetables are cooking, warm the tortillas and the black beans.
Assemble by adding the cooked vegetables to a tortilla, then topping with black beans and guacamole.
Notes
I use a mix of colors for appearance, though any bell peppers will work.
A large onion or 2 medium onions.
I find that 3/4 teaspoon salt is perfect (I use iodized salt). If you are more salt-sensitive, then I recommend starting with 1/2 teaspoon and then adding more salt once it's cooked.
This recipe makes enough for ~8 fajitas. I use flour tortillas but corn tortillas will also work.
I often warm a can of beans to serve inside the fajitas for added protein and fiber, but you can use cooked from dried black beans.