Cook green beans in large saucepan of salted, boiling water until crisp/tender, about 4 minutes. Drain and rinse with cold water. (Can be made ahead; cover and refrigerate until ready to serve.)
Heat 12-inch skillet over medium-high heat. Add prosciutto; saute 3 minutes or until crisp. Remove and reserve.
Reduce heat to medium-low; heat oil in skillet. Add onions; cook 10 minutes, stirring occasionally, until golden and translucent. If made ahead, let beans come to room temperature. Add to pan along with pepper and stir; cook 4 minutes or until heated. Stir in prosciutto.
