Place the beans in a steamer basket set 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 oil, vinegar, mustard, shallot, some salt and pepper. Pour the dressing over the beans and toss to coat. Cover and let marinate in the refrigerator overnight. Bring to room temperature and serve.
