Peel and chop the sweet potatoes and the carrots. Transfer to a roasting tin, and drizzle with 3 tbsp of olive oil. Add cumin seeds, ground black pepper and rosemary. Give it a good mix for the olive oil and the seasoning to coat the vegetables. Roast for 20 minutes or until tender.
In a pot, add 1 tbsp of olive oil and fry the onion until softened. Add garlic and stir for 1 minute before adding the stock.
Add vegetable stock, and simmer for 10 minutes. Let the roasted vegetables cool down, then add them to the pot.
Using a hand blender, blend the vegetables until soup is silky and smooth. Stir in milk, and add some salt.
Serve warm with crusty bread on the side, or croutons and good grinding of pepper.
If the soup turns out too thick, you can add more milk to reach the consistency that you want.