In a small bowl add sliced shallots, and vinegar. Let marinate for at least 1-3 hours. While beans are cooking mix mustard, salt and pepper into shallots while gradually whisking in olive oil.
Bring a large pot of salted water to boil. Or use your steamer according to settings.
Add the green beans and cook until tender crisp, 5-7 minutes depending on size. Transfer to a bowl of ice water to stop cooking. Let cool for a couple of minutes, then drain and pat dry. Put on serving platter.
Pour dressing over beans and toss to coat. Sprinkle on fresh basil and serve.