Cook beans according to package directions, substituting 1 cup broth for 1 cup water. Cook until tender but not mushy. Drain and place in large bowl.
Trim off root and all but 1" of green tope from the leeks. Slice in quarters lenghthwise, and wash thoroughly.
Place leeks in microwave safe dish with remaining broth, cover with plastic wrap, and cook on high for 4-5 minutes, until tender. Drain, reserving chicken broth.
Cut prosciutto lengthwise into narrow ribbons. Slice leeks crosswise into 1/8" slices. Saute leeks and garlic in olive oil until garlic is soft. Add the chopped sage, rosemary, and reserved broth and simmer until reduced slightly. Remove from heat and stir in prosciutto and lemon juice. Add to the beans and toss carefully with a wooden spoon. Season with salt and pepper.
Garnish with sage and peppers. (The salad may be refrigerated up to 24 hours ahead. Return to room temperature before serving).