How to Make It
In a small skillet, toast the walnuts over moderate heat until lightly golden and fragrant, about 5 minutes. Let cool, then coarsely chop.
Bring a large saucepan of salted water to a boil. Add the green beans and cook until crisp-tender, about 3 minutes. Drain and rinse under cold water until cool. Pat dry.
In a small skillet, heat 1 tablespoon of the olive oil. Add the prosciutto and cook over moderate heat until crisp, about 6 minutes. Drain on paper towels.
In a large bowl, whisk the remaining 1 tablespoon of olive oil with the vinegar and season with salt and pepper. Add the walnuts, green beans, prosciutto and Gruyère and toss to coat. Serve right away.