Preheat your oven to 200°C (400°F). Line a baking sheet with parchment paper for easy cleanup.
Peel the sweet potatoes and carrots, then chop them into even chunks to ensure they roast uniformly.
Toss the chopped vegetables with olive oil, salt, and pepper until well coated. Spread them out in a single layer on the prepared baking sheet.
Roast the vegetables in the oven for about 30-35 minutes, flipping halfway through, until they are caramelized and tender with golden edges.
Meanwhile, heat a splash of olive oil in a large pot over medium heat. Add the chopped onion and sauté for about 5 minutes until translucent and fragrant.
Add the minced garlic to the pot and cook for another minute until fragrant, being careful not to burn it.
Once the roasted vegetables are done, add them to the pot with the sautéed onions and garlic.
Pour in the vegetable broth and bring the mixture to a gentle boil. Reduce the heat and let it simmer for about 10 minutes to allow the flavors to meld.
Use an immersion blender directly in the pot, or carefully transfer the soup to a blender, and blend until completely smooth and velvety.
Taste the soup and adjust the seasoning with salt, pepper, and a squeeze of lemon for brightness if desired.
Serve the hot soup in bowls, garnished with fresh herbs or a drizzle of olive oil for an extra touch.