Preheat your oven to 200°C (390°F). Peel the sweet potatoes and carrots, then chop them into roughly 2-inch pieces for even roasting.
Toss the chopped vegetables with a tablespoon of olive oil, a pinch of salt, and pepper. Spread them out evenly on a baking sheet lined with parchment paper.
Roast the vegetables in the oven for about 30 minutes, until they are caramelized around the edges and fragrant with a sweet aroma.
Meanwhile, dice the onion and sauté it in a large pot with a tablespoon of olive oil over medium heat until it becomes translucent and releases a sweet aroma, about 5 minutes.
Once the roasted vegetables are ready, add them to the pot with the sautéed onion.
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 beautifully.
Remove the pot from heat and use an immersion blender or transfer the mixture to a blender to puree until smooth and velvety. Be careful when blending hot liquids.
Taste the soup and adjust seasoning with salt, pepper, and a squeeze of fresh lemon juice for brightness.
Serve the soup hot, garnished with herbs or a swirl of cream if desired, and enjoy the warm, earthy flavors.