01 - Preheat the oven to 425°F and line a large baking sheet with parchment paper.
02 - Remove green leaves and trim the stem from each cauliflower, keeping the core intact. Slice each head into two thick steaks, about 1 to 1½ inches thick. Reserve any loose florets for another use.
03 - Arrange the cauliflower steaks on the prepared baking sheet. Brush both sides evenly with olive oil, and season with sea salt, black pepper, and smoked paprika if using.
04 - Roast the steaks for 20 minutes. Carefully flip and continue roasting for an additional 10 to 12 minutes until golden brown and fork-tender.
05 - While the 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. Stir well and let rest for at least 10 minutes to develop flavors.
06 - Serve the roasted cauliflower steaks immediately, drizzled generously with chimichurri sauce.