Bring in a pan 3 l of lightly salted water to boil over high heat. Drop string beans and bring back to a boil. Lower the heat and cook for 10 minutes, until the beans are tender. Drain the beans, wash them under cold water. Melt the butter in a heavy skillet set over high heat. Add the onions and green pepper, lower the heat and cook for 10 minutes, or until the vegetables are tender. Stir in the tomatoes and basil, raise the heat to high, and boil rapidly for 1-2 minutes, until most of the juices in the pan have evaporated.
Stir in the green beans and simmer until heated through.
In a mixing bowl, beat together the egg, sour cream, salt and a few grindings of black pepper. Stir in the vegetables. Serve at once.