Preheat the oven to 425°F. Line a large baking sheet with parchment paper.
Place sweet potatoes and carrots on the baking sheet. Toss with 1 tablespoon olive oil, ½ teaspoon salt, and ¼ teaspoon black pepper.
Roast for 25 to 30 minutes, stirring once halfway through, until the vegetables are tender and lightly browned.
While the vegetables roast, heat the remaining 1 tablespoon olive oil in a large soup pot over medium heat.
Add diced onion and cook for 5 to 7 minutes, until soft and translucent.
Add garlic and ginger. Cook for 30 seconds, stirring constantly until fragrant.
Add curry powder, cumin, smoked paprika, and optional cayenne. Stir for 30 seconds to toast the spices lightly.
Add roasted sweet potatoes, carrots, and vegetable broth to the pot. Bring to a gentle simmer and cook for 5 to 10 minutes.
Remove the pot from the heat. Blend the soup until completely smooth with an immersion blender. If using a countertop blender, blend carefully in small batches.
Return the soup to low heat. Stir in coconut milk, the remaining salt and black pepper, and lime juice if using.
Add more broth if you prefer a thinner consistency. Taste and adjust seasoning as needed.
Serve hot with toasted pumpkin seeds, herbs, coconut milk, Greek yogurt, croutons, or your preferred toppings.