01 - Preheat the oven to 425°F. Line a baking sheet with parchment paper.
02 - Remove outer leaves and trim stems of cauliflower, keeping the core intact. Place each head stem-side down and slice into 1-inch thick steaks, yielding 2 to 3 per head. Reserve loose florets for another use.
03 - Arrange cauliflower steaks on the prepared sheet. Brush both sides evenly with olive oil, then sprinkle smoked paprika, garlic powder, black pepper, and sea salt.
04 - Roast steaks for 20 minutes, carefully flip, then roast an additional 12 to 15 minutes until golden and tender.
05 - While roasting, combine parsley, cilantro, 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 flavors meld for at least 10 minutes.
06 - Plate roasted cauliflower steaks and spoon chimichurri generously over each. Serve immediately.