PREHEAT the oven to 450°F.
LINE a large rimmed baking pan with parchment paper. Combine the sweet potatoes, onion, and carrots in a large bowl. Drizzle with 3 tablespoons of the olive oil; toss to coat. Sprinkle with the salt, pepper, and cumin seeds; toss to combine.
TRANSFER to the baking pan. Bake for 20 to 25 minutes, until tender and beginning to brown.
HEAT the remaining olive oil in a large pot over medium heat. Add the garlic, ginger, and jalapeño; cook for 1 minute. Stir in the broth, coconut milk, lemon juice, and coriander; bring to a boil. Carefully add the roasted vegetables. Remove from the heat.
USE an immersion blender to blend the soup in the pot. (Or let the soup cool briefly, then carefully transfer to a blender in batches. Cover and pulse a few times, then blend until smooth.)
STORE leftovers in the fridge for up to 4 days, or in the freezer for up to 3 months.