Place the beans in a steamer in a pot filled with 2 inches of boiling water. Cover and steam until the beans are bright green and still slightly firm to the bite, about 10 minutes.
Meanwhile, whisk together the vinegar, mustard, shallot, some salt and pepper, and the oil. Pour the dressing over the beans and toss to coat. Cover and let marinate in the refrigerator for at least 1 hour or up to overnight. Bring to room temperature and serve.