Bring the water to a boil in a medium-sized saucepan. Trim and wash the beans, remove any blemished ones, place them in the boiling water, and cook for about 5 minutes, or until they are tender but still slightly crisp. Drain the beans, place them in a glass salad bowl, and allow them to cool. Place the onion slices in a small bowl, cover them with the lemon juice, and allow them to soak for 30 minutes. When ready to serve, add the onions to the string beans, drizzle them with olive oil, season with salt and pepper, and serve at room temperature.