Preheat the oven to 425°F and line a rimmed baking sheet with parchment paper.
Place the cauliflower, 1 tablespoon olive oil, ½ teaspoon salt and pepper on the prepared baking sheet and use your hands to toss to combine. Bake for 20-25 minutes, or until the cauliflower softens and becomes golden brown and caramelized on the edges.
Meanwhile, heat the remaining 1 tablespoon oil in a large pot over medium-high heat. Add the onion, garlic, ginger, remaining 1 teaspoon salt, coriander, turmeric and cumin. Cook for 5 minutes, until the onions soften and become fragrant.
Add the sweet potatoes, diced tomatoes and vegetable broth. Bring to a boil over high heat, then reduce to a simmer, and continue cooking uncovered for 35-40 more minutes, stirring occasionally, until the sweet potatoes are tender.
Pour the coconut milk and most of the roasted cauliflower (save some for topping) to the pot with the sweet potatoes. Blend with an immersion blender until smooth and creamy.
Ladle into bowls, garnish with the reserved cauliflower florets and cilantro, and serve with crusty bread, if desired.