Melt the butter in a large pan, add the onion and fry for 3 minutes, stirring. Add 300 ml (1/2 pint) 1 1/4 cups of vegetable stock to the pan, with the chopped tomatoes and bicarbonate of soda (baking soda). Bring the soup to the boil and simmer for 20 minutes.

Remove the pan from the heat and set aside to cool. Puree the soup in a blender or food processor and pour through a fine strainer back into the saucepan.

Add the remaining vegetable stock and the fusilli to the pan, and season to taste with salt and pepper.

Add the sugar to the pan, bring to the boil, then lower the heat and simmer for about 15 minutes.

Pour the soup into a warm tureen, swirl the double (heavy) cream around the surface of the soup and garnish with fresh basil leaves. Serve immediately.