1 1/2 pounds new potatoes (about 15), halved or quartered if large
1/2 cup chopped fresh herbs (such as flat-leaf parsley, basil, and chives)
2 scallions, thinly sliced
1/2 red pepper, diced
2 tablespoons olive oil
1 tablespoon whole-grain mustard
1 tablespoon red wine vinegar
kosher salt and black pepper