Fill a large heavy saucepan two-thirds full with salted water and bring to a boil. Add the green beans and blanch just until they are crisp-tender, 4 to 6 minutes. Transfer to an ice bath and set aside to cool. When the beans are cool, transfer them to a colander to drain.
While the beans are draining, make the vinaigrette. In a large stainless steel or other nonreactive bowl, whisk together the vinegar, mustard, and salt. As you continue to whisk, add the oil in a thin, steady stream until the vinaigrette is emulsified. You may not need all the oil, depending on the acidity of the vinegar you are using. Season with pepper.
Arrange the drained beans in a large shallow bowl or serving dish and scatter the sliced onion on top. Arrange the egg slices on top of the onion and pour the vinaigrette over all. The salad can be served immediately, or it can marinate in the vinaigrette, covered with plastic wrap and refrigerated, for up to several hours.
Makes 8 servings