Preheat the oven to 400°F.

Trim the tough outer leaves from the cauliflower, cut the stem level, and cut an X into the bottom. Rub the cauliflower with olive oil and season generously with salt and pepper.

Stir together the coconut milk and harissa paste. Place the cauliflower upside down in a small Dutch oven or other oven-safe vessel with a lid. You want to use a pan that is not too large. Pour the coconut milk mixture over the cauliflower, cover the pan, and bake for 30 minutes. Flip the cauliflower over and baste with the sauce. Cover and cook for another 30-45 minutes or until the cauliflower is tender. Transfer to a serving plate, drizzle with the sauce, and sprinkle with the pistachios and mint.