Preheat the oven to 425°F (220 ̊C).
Spread the cauliflower orets on a baking sheet. Drizzle them generously with oil, season with salt, and toss to coat. Roast for 15 minutes, tossing the cauli ower halfway through. Continue to roast until golden brown, about 15 minutes more.
While the cauliflower is roasting, chop the leeks crosswise into roughly 1⁄4-inch (6-mm) slices. In a medium saucepan, heat the oil and thyme over medium heat and sauté the leeks until they are slightly softened, about 2 minutes. Add the garlic and cook until soft and fragrant, about 2 minutes more.
Add the potatoes, stock, cream, 2 cups (480 ml) water, and the roasted cauli ower to the pot. Bring the mixture to a gentle simmer over medium-low heat and cover, cooking until the pota-toes are fork tender, 15 to 20 minutes. Once the potatoes are tender, remove the thyme stems from the mixture (the leaves should have fallen o during cooking).
Transfer the mixture to a blender and cover the hole of the blender top with a towel. Blend until the mixture is very smooth. Stir in the lemon juice and season with 2 teaspoons salt and some pepper. Divide the soup among four bowls and top it with lemon zest, a swirl of the watercress pesto, and a sprinkle of dukkah.
