Cook the beans in a little water in a small saucepan for 4 to 5 minutes or until crisp-tender. Drain and keep warm.
Heat the oil in a medium frying pan and fry the prosciutto briefly over moderate heat until crisp. Remove the prosciutto and put aside.
Cook the onion and garlic in the same pan for 2 minutes or until translucent. Cut the prosciutto into 2 cm pieces, return to the pan along with the parsley and basil, then pour the dressing over the beans. Tip into a bowl and serve hot or at room temperature.