Line a baking tray with aluminium foil. Toss the cauliflower with 2 tablepsoons olive oil, cumin seeds, turmeric powder, and harissa until evenly coated. Spread in a single layer and bake at 180°C/360°F for 45 to 50 mins, or until they’ve browned. Remove and set aside. Taste and check the fieriness of the harissa at this stage; add more if you'd like and return to the oven for 5 more minutes.
Pour 1 tablespoon of olive oil in a pot and sauté the onion and garlic until translucent. Add the roasted cauliflower along with any of the remaining cumin on the foil to the pot, and pour in 2 cups of water. Season with pepper and salt. Cover and bring to the boil, then simmer for 10 to 15 minutes, until the florets and tender. Take off the heat and let it cool slightly.
Once cooled, add to a blender/use an immersion blender and blend until smooth. Check seasoning and add milk and more water to get the right consistency. Return the soup to a clean pot and gently reheat. Best served hot.