Heat oven to 220C/200C fan/ gas 7 and put the sweet potatoes and carrots into a large roasting tin, drizzled with 2 tbsp olive oil and plenty of seasoning. Roast the veg in the oven for 25-30 mins or until caramelised and tender.
Meanwhile, put the remaining 1 tbsp olive oil in a large deep saucepan and fry the onion over a medium-low heat for about 10 mins until softened. Add the garlic and stir for 1 min before adding the stock. Simmer for 5-10 mins until the onions are very soft, then set aside.
Once the roasted veg is done, leave to cool a little, then transfer to the saucepan and use a hand blender to process until smooth. Stir in the crème fraîche, a little more seasoning and reheat until hot. Serve in bowls topped with a swirl of crème fraîche and a good grinding of black pepper.