01 - Preheat the oven to 425°F and line a baking sheet with parchment paper.
02 - Remove the outer leaves and trim the stem of each cauliflower head while keeping the core intact. Position the cauliflower stem-side down and slice into 1-inch thick steaks using a large knife; expect 2–3 steaks per head. Reserve any loose florets for another use.
03 - Arrange the cauliflower steaks on the prepared baking sheet. Brush both sides evenly with olive oil, then sprinkle with smoked paprika, garlic powder, black pepper, and sea salt.
04 - Roast cauliflower steaks in the oven for 20 minutes. Carefully flip them and continue roasting for an additional 12 to 15 minutes until golden and tender.
05 - While roasting, combine parsley, cilantro, minced garlic, oregano, olive oil, red wine vinegar, crushed red pepper flakes if using, sea salt, and black pepper in a bowl. Stir well and let rest for at least 10 minutes to allow flavors to meld.
06 - Place roasted cauliflower steaks on plates and spoon chimichurri generously over each. Serve immediately.