Preheat the oven to 200 degrees Celsius. Cut the half cauliflower into thick, sturdy slices to create cohesive steaks. Place them on a baking sheet, brush both sides with olive oil, and season with salt and pepper.
Slide the baking sheet into the oven and roast the cauliflower steaks for 25 minutes until tender and golden brown. Meanwhile, make the chimichurri by finely chopping the fresh parsley, oregano, and garlic clove. Mix this in a bowl with the red pepper flakes, red wine vinegar, and a generous splash of olive oil.
Cut the halloumi into equal slices. Heat a grill pan on the stove and cook the cheese on both sides for a few minutes until crispy and golden brown.
Remove the roasted cauliflower steaks from the oven and place them directly on plates. Arrange the warm halloumi over the vegetables and generously spoon the prepared chimichurri over the top.