01 - Preheat oven to 425°F and line a large baking sheet with parchment paper.
02 - Remove green leaves and trim stem from cauliflower heads, keeping cores intact. Slice each head into two thick steaks about 1 to 1½ inches thick. Set aside any loose florets for other uses.
03 - Place cauliflower steaks on the baking sheet. Brush both sides with olive oil and season evenly with sea salt, black pepper, and smoked paprika if using.
04 - Roast for 20 minutes, then carefully flip steaks and continue roasting for 10 to 12 minutes until golden brown and tender.
05 - While cauliflower roasts, combine parsley, cilantro (if using), garlic, oregano, olive oil, red wine vinegar, red pepper flakes, sea salt, and black pepper in a bowl. Mix thoroughly and let stand for at least 10 minutes to meld flavors.
06 - Serve hot roasted cauliflower steaks drizzled generously with chimichurri sauce.