01 - Preheat the oven to 425°F. Line a baking sheet with parchment paper.
02 - Remove the leaves and trim the stem ends from each cauliflower, keeping the core intact. Slice each cauliflower vertically into 1-inch-thick steaks. You should get 2-3 steaks per head; save extra florets for another use.
03 - Arrange the cauliflower steaks on the prepared baking sheet. Brush both sides with olive oil and sprinkle with smoked paprika, garlic powder, salt, and pepper.
04 - Roast for 25-30 minutes, flipping carefully halfway through, until the cauliflower is golden and fork-tender.
05 - In a bowl, combine parsley, cilantro, garlic, shallot, oregano, red pepper flakes, olive oil, red wine vinegar, salt, pepper, and lemon juice. Mix well and let sit for at least 10 minutes to develop flavor.
06 - Transfer roasted cauliflower steaks to serving plates and generously spoon chimichurri sauce over the top. Serve immediately.