1. 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.
2. 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.
Wine Recommendation: Alsace is known for its delicious white wines, but an excellent red, pinot d'Alsace, is made from pinot noir as well. It's worth looking for to create an intriguing regional pairing. If you can't find one, a fruity Beaujolais will do just fine.