Cook the beans in fresh cold water to cover over low heat until tender but not mushy, about 45 minutes. The beans tend to firm up a bit after cooking, so cook them to the soft side of tender rather than the firmer side. Drain the beans and transfer to a large mixing bowl. Toss with enough olive oil to coat, then chill.
Add the remaining oil, the onion, and vinegar to the beans and toss to coat and combine. Season to taste with salt and pepper. I like these a bit peppery. Sprinkle with the prosciutto and eggs. Garnish with parsley and tomato wedges if you like.
© 1998 Joyce Goldstein. All rights reserved.