Place the celeriac in a roasting tin and brush liberally with oil. Cook for about forty five minutes or until tender and browned. Meanwhile, in a large pot, fry the onion and carrot in a little oil for five minutes on a gentle heat, add the garlic and cook for a further minute. Pour in the wine, reduce by three quarters and pour in the stock. Let it simmer whilst you make the oil. Wiz the watercress, basil, oil, lime, zest, sea salt and garlic in a food processor for about a minute. Pour into a screw top jar and set aside. Turn off the stock and wait till the celeriac is cooked. Add it to the soup and cook for a further five minutes. Stir in the cream, liquidise the soup and pass through a sieve if you want a really smooth soup. Ladle into bowls and drizzle over the watercress oil.