Preheat Oven and Prep Baking Sheet - Preheat your oven to 400°F (205°C). Line a baking sheet with parchment paper or coat with cooking spray. - Make sure your oven is fully preheated before placing the cauliflower on the baking sheet to ensure even cooking.

Prepare the Cauliflower - Remove the leaves and stem from the cauliflower, so you are left with just the head. Place the cauliflower core-side down on a cutting board and slice it in half. Then, slice each half in 1/2-inch (1 cm) thick 'steaks'. - If you're having difficulty slicing the cauliflower 'steaks', you can also use your hands to break the cauliflower into large bite-sized florets instead.

Mix the Spices and Oil - In a mixing bowl, whisk together the olive oil, harissa, smoked paprika, garlic powder, and salt. - Make sure to whisk the mixture well to ensure each cauliflower steak is evenly coated in the spiced oil.

Coat the Cauliflower with Spiced Oil - Place each cauliflower steak in the mixing bowl of spiced oil and use a spoon or brush to generously coat each one. - It's important to make sure each cauliflower steak is coated well with the spiced oil to ensure maximum flavor and crispiness.

Roast the Cauliflower Steaks - Place the cauliflower steaks on the prepared baking sheet and roast in the oven for 20-25 minutes, flipping once halfway through cooking, until the cauliflower is tender and both sides are browned and crispy. - Make sure to flip the cauliflower steaks gently halfway through cooking to avoid breaking them apart.

Serve and Enjoy - Serve the cauliflower steaks immediately, garnished with fresh herbs or a squeeze of lemon juice if desired. - These spicy harissa cauliflower steaks can be served as a main course with a side salad or rice, or as a side dish with grilled chicken or fish.