In a bowl, whisk together the vinegar, mustard, oil, salt and pepper and set aside.
Chop the romaine lettuce and place in a large bowl. Cut the tomatoes into wedges and the red onion into thin slices and add to the bowl. Just before serving, toss the salad with the mustard vinaigrette.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.