Peel and finely chop the onion. Puree the tomatoes and 1/4 litre stock and pass through a sieve. Heat the fat in a pot and fry the onions until transparent. Add the tomatoes and the remaining stock and season with salt and pepper. Bring the soup to the boil, cover and simmer for about 5 minutes. Add the sauce thickener while stirring, bring to the boil again briefly and season to taste with salt and pepper.
Wash basil, dab dry and cut into strips except for a little bit for garnishing. Puree basil in 100 g cream. Add the rest of the cream and whip until creamy with the whisk of the hand mixer. Arrange tomato soup and basil foam in deep plates and serve garnished with basil. Serve with baguette