Preheat your oven to 200 C / 180 C fan oven / 390 F.
Peel the sweet potatoes and cube them in bite sized pieces. Add them to a baking tray, and drizzle 1 teaspoon olive oil on top. Move the cubes around using tongs or your hands, and make sure that each piece of sweet potato is coated completely.
Roaste the sweet potatoes in the oven until they start to turn golden on the edges. They should also be soft enough to put a fork through them with little resistance. This takes 20-30 minutes, depending on the size of your potato chunks.
When the potatoes are almost done roasting, you can start to prepare the soup. Heat 1 teaspoon olive oil in a large pot, and add finely chopped onion and garlic. Allow these to saute for a few minutes, until the onion is soft and translucent. Stir occasionally to prevent it from burning.
Add curry paste to the pot, stir to combine it with the onions, and allow it to cook for a minute or two while stirring. This will help bring out the flavour for the rest of the soup.
Add the coconut milk and vegetable stock, bring it to a boil and leave it to simmer for a few minutes. Add the roasted sweet potatoes, and leave the soup to simmer for another 5 minutes.
Use an immersion blender or regular stand blender to blend the soup completely smooth.
This soup will be very thick and creamy. You can make it thinner by adding some water or vegetable stock until it reaches your desired consistency.
Lastly, add salt and pepper to taste before serving.