With a mortar and pestle, grind together rosemary and sea salt. Pour in olive oil and continue to mash until the oil starts to turn a darker green. Set aside for about an hour.
Preheat oven to 220 C / Gas 7. Coat a shallow roasting tin with cooking oil spray.
Place the squash, swede, parsnips and potatoes in a large bowl. Pour the olive oil mixture through a fine-mesh sieve into the bowl. Toss vegetables with oil to coat.
Evenly spread vegetables on the prepared tin. Roast 30 minutes in the preheated oven, until nicely browned and cooked through. You may need to remove some of the smaller pieces from the oven before all of the vegetables are cooked to avoid burning them.
While the vegetables are roasting, simmer chicken stock in a large saucepan over medium low heat. When the vegetables are done add them to the simmering chicken stock and simmer together for about 10 minutes.
Use an hand blender to puree the soup in the saucepan or puree in batches in a liquidiser or food processor. Add extra stock or water if the soup is too thick. Season with salt and pepper to taste. Serve in warmed bowls, garnishing each serving by drizzling a quarter teaspoon of balsamic vinegar and a quarter teaspoon of white truffle oil over the soup.