Preheat oven to 450°F. Line a rimmed baking sheet with parchment paper.
Combine coriander, cumin, cinnamon and turmeric in a large bowl. Add the cauliflower to the spices along with 1 tablespoon oil and toss to combine. Transfer the cauliflower to the prepared baking sheet and roast until the edges are browned, 15 to 20 minutes. Set aside.
Meanwhile, heat the remaining 1 tablespoon oil in a large pot over medium-high heat. Add onions and carrots and cook until the onions soften, about 3 to 4 minutes. Add the garlic and ginger and cook until fragrant, about 1 more minute.
Stir in diced tomato tomatoes, vegetable broth and sweet potatoes. Cover and bring to a boil over high heat. Reduce heat to maintain a gentle simmer and cook, partially covered and stirring occasionally, until the vegetables are tender, about 35 to 40 minutes.
Stir in coconut milk and the roasted cauliflower. Blend with an immersion blender until smooth and consistent.
Serve warm with roasted chickpeas, if desired.