Source: Martha Stewart, Everyday Food - July/August 2009
adapted by http://theresalwaysthyme.blogspot.com
2 1/2 pounds fingerling or small new potatoes, halved (quartered if large)
Coarse salt and ground pepper
1/4 cup olive oil
3 tablespoons Dijon mustard
2 tablespoons sherry vinegar
1 small shallot, minced (about 2 tablespoons)
3 tablespoons chopped fresh parsley
1 teaspoon chopped fresh thyme
1/4 small red onion, sliced thin
Place potatoes in a large pot; cover with cold water by 1 inch and season generously with salt. Bring to a boil; reduce to a simmer and cook until potatoes are tender, about 15 minutes. Run under cold water to cool slightly, then drain.
In a large bowl, whisk together oil, mustard, vinegar, shallot, parsley, and thyme; season with salt and pepper. Add potatoes and onion and toss to combine. Serve at room temperature.
To make ahead, you can refrigerate and store overnight.