• Chopped parsley for garnish, optional
Preheat oven to 400 degrees. Remove and discard the green leaves of the cauliflower, cut the florets and the stem into 2-inch pieces. In a large bowl, toss the cauliflower with enough of the oil to lightly coat. Then toss with the cumin, coriander, ginger, turmeric and a little salt and pepper. Spread out on a baking sheet so that the pieces do not touch. Roast until the cauliflower is lightly browned and cooked through, about 25 minutes. Set a few roasted florets aside to garnish the soup if you wish.
Heat the butter or oil in a large saucepan or Dutch oven set over low heat. Add the leeks and garlic and sauté until fragrant, about 3 minutes. Stir in the potato and the stock and bring to a simmer and cook until the potato is tender, about 10 minutes. Add the roasted cauliflower and simmer until the flavors come together, about 5 minutes. Purée the soup in batches using a blender or use an immersion blender directly in the pot. Taste and adjust the seasonings. Serve the soup garnished with the reserved florets, if you wish.