Step 1: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper.
Step 2: Prepare the cauliflower: Remove the outer leaves from the cauliflower head. Trim the very bottom of the core, but leave enough of the core intact to hold the 'steaks' together.
Step 3: Slice the cauliflower: Place the cauliflower stem-side down on a cutting board. Using a large, sharp chef's knife, slice the cauliflower vertically into 3/4 to 1-inch thick 'steaks'. You should get 2-4 good steaks from a large head. Any remaining florets can be roasted alongside the steaks.
Step 4: Season the cauliflower: Arrange the cauliflower steaks and any loose florets in a single layer on the prepared baking sheet. Drizzle generously with 3 tablespoons of olive oil, then season evenly with salt and black pepper.
Step 5: Roast the cauliflower: Roast in the preheated oven for 20-25 minutes. Flip the steaks and continue to roast for another 15-20 minutes, or until the cauliflower is tender-crisp and beautifully golden brown with caramelized edges.
Step 6: Prepare the chimichurri: While the cauliflower roasts, combine the finely chopped parsley, minced garlic, red wine vinegar, red pepper flakes (if using), and dried oregano (if using) in a small bowl.
Step 7: Finish the chimichurri: Slowly drizzle in 1/4 cup of extra virgin olive oil, whisking constantly until the mixture is well combined. Taste and adjust seasonings as needed, adding more salt, pepper, or vinegar to your preference. Let the chimichurri sit at room temperature for at least 15 minutes for the flavors to meld.
Step 8: Serve: Once the cauliflower steaks are roasted, carefully transfer them to a serving platter. Spoon a generous amount of the fresh chimichurri over each steak and any roasted florets. Serve immediately.