Turn the oven to 400 degree F. Place a roasting pan with the water at the bottom of the oven. The water will help the cauliflower steam and ensure even cooking. 

First, make the spice paste. Just add the curry powder, harissa paste, and the oil in a bowl. Take a taste test to see if the salt is sufficient. Add salt only if you think you will need it.

Prepare the cauliflower by cutting base. Make sure that the head sits flat on a plate. 

Now apply the paste all over the cauliflower. Start with the bottom. Flip over the cauliflower and apply 2/3 of the spice mix. Spoon the mix through the gaps between the florets at the bottom. This will ensure that most of the cauliflower is infused with the spices. Turn over and apply the paste at the top. 

Place the cauliflower head in a skillet that has a light of the oil. Place this on a rack above the water pan. Let the cauliflower roast for 35-40 minutes. A large cauliflower will naturally take more time than a smaller one. The duration will also depend on how soft you want your vegetable to be. You may need to roast for longer if you want it very soft. 

Turn the oven to high broil at the last 2-3 minutes for browning the top. Remove the water pan before that. 

When serving, just transfer this cauliflower to a beautiful serving plate. Some of the spice and vegetable juices may come out at the bottom of the skillet. Add this to the serving plate as well. You can also scatter some parsley or coriander leaves on top. For a large vegetable platter, serve it surrounded with other grilled vegetables.