Preheat oven to 400 degrees. Line a large baking sheet with parchment paper or foil and set aside.: When the oven reaches 400 degrees you will notice a dry, warm breath as you open it, and that steady heat is what will coax sugars out of the cauliflower so it caramelizes rather than steams. Be sure your baking sheet is large enough so the florets lie in a single layer, otherwise they will steam and become limp. A common mistake is crowding the pan, which prevents browning. Use parchment or foil to make cleanup effortless and to prevent sticking, and preheating ensures the moment you place the tray in, roasting begins immediately.
In a small bowl, stir together chili powder, cumin, smoked paprika, garlic powder, and onion powder.: As you mix these spices the aroma will bloom, releasing warm, earthy notes that hint at the final flavor. Combining them first helps you distribute the seasoning evenly across the cauliflower , so each piece gets a balanced coating. If you skip this step you'll risk uneven pockets of flavor. Taste a pinch if you like, and adjust subtly; these spices should support, not dominate, the chimichurri sauce .
Place the cauliflower in a large bowl and drizzle with olive oil. Toss until well coated. Pour the seasoning mixture over the cauliflower and stir until well coated.: The sound here is gentle as the florets rub together and the oil carries the spices, creating a sheen on each piece. The oil is what allows the spices to adhere and helps promote even browning in the oven. Make sure every floret has some oil and seasoning, because dry spots will roast without color. Avoid adding too much oil, which leads to soggy texture, and resist the urge to crowd the bowl while tossing to keep the coating consistent.
Place the cauliflower on the prepared baking sheet in an even layer. Roast for 25 to 30 minutes or until cauliflower is tender. Remove from the oven.: During roasting you will first notice a toasted scent, then hints of sweetness as the edges begin to brown and caramelize. Flip the florets halfway if you want more even browning, but not constantly because the oven needs time to form those golden pockets. Look for edges that are golden to light brown and a tender interior when pierced with a fork. A typical error is pulling them too early, leaving them undercooked and lacking flavor depth. Once out of the oven, let the cauliflower rest briefly; carryover heat finishes the cook and concentrates the aroma.
To assemble the tacos, top a tortilla with roasted cauliflower, beans, chimichurri sauce, cabbage, pickled red onions, cilantro, and avocado. Serve immediately.: The moment of assembly is where textures come together, and you should feel the warmth of the roasted cauliflower against the soft, slightly charred corn tortilla . Spoon warm, rinsed black beans onto the tortilla, then add a generous drizzle of chimichurri sauce so its brightness cuts through the roasted notes. Add crisp red cabbage and tangy pickled red onions for contrast, sprinkle chopped cilantro , and finish with chopped avocado for creaminess. Serve immediately so the tortillas remain pliable and the contrast between warm filling and cool toppings is at its best. A common misstep is assembling too early and letting ingredients make the tortillas soggy; assemble when you are ready to eat.