1. Heat the buter with the oil in a large saucepan over moderate heat. Add the onion and cook, stirring occasionally, for 5 minutes.

2. Stir in the tomatoes and tomato paste, and cook for 2 -3 minutes. Blend in the flour, then add the chicken stock, sugar, salt and pepper. Cover and simmer for 15 minutes.

3. Puree the mixture in a blender or food processor, then return the puree to the saucepan and add the cream. Reheat and simmer, uncovered, for 2-3 minutes.