Adding 1/8 teaspoon of dry mustard (like Colman’s) to a cup of vinaigrette before you whisk it together helps the emulsion of oil and vinegar from breaking apart. It also adds a small spark of flavor. Any prepared mustard adds flavor to a vinaigrette, but you need dry mustard for the emulsion trick. Another trick: Mix the vinaigrette in a blender instead of whisking. It won’t separate for a day!
Learn more about vinegar in our article, Vinegar 101.