Bring a cupful of vegetable broth to a boil. Trickle, stirring, 0.5 cup of the same cold broth shaken with 2 tea-spoons of flour into it. Bring to a boil. When the resulting broth thickens, add 1 table-spoon of butter, sour cream, and tomato paste. Heat without bringing to a boil. Instead of tomato paste you can use fresh tomatoes. Cook them in butter and rub through a sieve.