Preheat your oven to 200°C (400°F). Chop the sweet potatoes into roughly 1-inch cubes and slice the carrots into rounds. Toss them with a tablespoon of oil and spread evenly on a baking sheet. Roast for 25-30 minutes until caramelized and fragrant.
While the vegetables roast, peel and chop the onion. In a large pot, heat a tablespoon of oil over medium heat. Sauté the onion until translucent and soft, about 5 minutes, until it starts to smell sweet and turns a light golden color.
Once the roasted vegetables are ready and cooled slightly, add them to the pot with the sautéed onions. Pour in the vegetable broth and bring the mixture to a gentle simmer. Let it cook for about 10 minutes to allow flavors to meld.
Use an immersion blender directly in the pot, or carefully transfer the soup in batches to a blender. Blend until the soup becomes completely smooth and velvety, with no chunks remaining. Be cautious of hot splashes when blending.
Taste the soup and season with salt and pepper. Add a splash of citrus juice or zest to brighten the flavors, stirring well to incorporate.
Allow the soup to rest for 10 minutes off the heat, giving the flavors time to deepen and balance. Then, give it a final stir and check the seasoning again.
Serve the soup hot, garnished with a drizzle of cream, herbs, or a sprinkle of extra citrus zest if desired. Enjoy the comforting, earthy goodness in every spoonful.