Preheat your oven to 400°F (200°C) and spread the chopped sweet potatoes and carrots evenly on a baking sheet. Drizzle with one tablespoon of olive oil and toss to coat. Roast in the oven for about 25 minutes, until the vegetables are tender and slightly caramelized around the edges.
While the vegetables roast, heat the remaining tablespoon of olive oil in a large pot over medium heat. Add the diced onion and cook, stirring occasionally, until it becomes translucent and fragrant, about 5 minutes. Then, stir in the minced garlic and cook for another minute until it smells sweet and aromatic.
Once the roasted vegetables are ready, add them directly into the pot with the sautéed aromatics. Pour in the vegetable broth and bring the mixture to a gentle boil over high heat. Reduce the heat to low, cover, and simmer for about 10 minutes to allow flavors to meld.
Remove the pot from heat and carefully blend the soup until smooth using an immersion blender. Alternatively, transfer the soup in batches to a blender, blending until silky and creamy. Be cautious with hot liquids—blend in small batches if using a regular blender.
Return the blended soup to low heat. Stir in the nutmeg or cinnamon, and season with salt and pepper to taste. Let it simmer gently for another 5 minutes, stirring occasionally, to deepen the flavor.
Serve the soup hot, garnished with a drizzle of olive oil, a dollop of yogurt, or fresh herbs if desired. Enjoy the rich, velvety texture and warm, sweet aroma in each spoonful.