Preheat your oven to 425°F 220°C (425°F).
Place the prepared cauliflower on one baking sheet and the leeks on another.
Toss each with 1 Tbsp 15 ml of olive oil.
Sprinkle the cauliflower with cumin, onion powder, garlic powder, and cayenne, then toss to coat evenly.
Place both baking sheets in the oven.
Roast until the vegetables are soft and lightly browned around the edges, about 10 minutes for the leeks and 20 minutes for the cauliflower.
Stir each sheet once, halfway through cooking.
In a large pot or Dutch oven, combine the potatoes, celery, garlic, thyme, vegetable broth, salt, and white pepper.
Add the roasted leeks and all but ½ cup 120 ml (4.1 fl oz) of the roasted cauliflower (reserve the rest for garnish).
Bring the soup to a simmer and cook, covered, until the potatoes and celery are tender, about 15 minutes.
Carefully transfer the soup to a blender or food processor.
Add the yogurt and purée until completely smooth, working in batches if necessary.
Ladle the soup into bowls and garnish with the reserved roasted cauliflower florets and fresh cilantro before serving.