Cut cauliflower into florettes. Peel potatoes, cut into 2-3cm pieces. Clean leek and chop. Peel garlic clove and push down on it with flat side of the knife to bruise. Place cauliflower, potatoes, leek, garlic and the rosemary sprig in an oven-proof dish. Drizzle with the olive oil and season with salt and pepper.
Place dish in the oven and roast until the potatoes and cauliflower have softened and are slowly turning golden. Don't let them brown too much, otherwise your soup will have dark burnt bits in it. If need be, cover the dish with aluminium foil.
Once the cauliflower and potatoes have softened, take the dish out and place vegetables in a saucepan. Remove the wooden stalk from the rosemary sprig.
Add vegetable stock and bring to the boil. Puree, add parmesan, milk, nutmeg and cumin. Make sure parmesan is melted and stirred through. Taste and season with salt and pepper if necessary.
I served the soup with some grated parmesan and leftover naan bread.