How to Make It
Put the onion in a small bowl of cold water and let stand for 10 minutes. Drain the onion, rinse, and then pat dry with paper towels.
Meanwhile, in a large glass or stainless-steel bowl, whisk together the mustard, vinegar, salt, and pepper. Add the oil slowly, whisking. Add the ham, cheese, onion, and parsley to the vinaigrette and toss.
Variations This is a fine place to use cold cuts or leftover meat. Thin strips of pork, lamb, beef, salami, or mortadella could all be used in place of the ham. Or, for you meat lovers, try one of them in place of the cheese.