Bring ¾ l of lightly salted water to the boil and cook the vegetables in it for 3-4 minutes. Strain the vegetables, catch the cooking water and set aside.
Sweat the flour in butter until lightly. Pour in the cooking water, stir well with the whisk and simmer over a medium heat for 3-4 minutes, stirring again and again. Mix about half of the vegetables into the soup, reserve the rest of the vegetables for the garnish. Let the soup simmer on a low flame for about 10 minutes, then stir the cream and the chopped basil into the soup.
Bring the soup to the boil and puree it finely with a hand blender. Add the retained vegetables, heat the soup again and season with salt and pepper.