01 - Preheat oven to 425°F. Line a baking sheet with parchment paper.
02 - Remove outer leaves and trim stem of cauliflower, keeping the core. Slice each head into 1-inch thick steaks, yielding about 2-3 steaks per head. Reserve loose florets.
03 - Place cauliflower steaks on the baking sheet. Brush both sides evenly with olive oil, then season with smoked paprika, garlic powder, black pepper, and sea salt.
04 - Roast for 20 minutes, carefully flip each steak, and roast an additional 12 to 15 minutes until golden and tender.
05 - Combine parsley, cilantro, garlic, oregano, olive oil, red wine vinegar, crushed red pepper flakes (if using), salt, and black pepper in a bowl. Stir thoroughly and let rest for at least 10 minutes to develop flavors.
06 - Arrange roasted cauliflower steaks on plates. Generously spoon chimichurri over each steak and serve promptly.