1 onion, cut into paper-thin slices
2 1/2 teaspoons grainy Dijon mustard
2 teaspoons red- or white-wine vinegar
1/4 teaspoon salt
1/4 teaspoon fresh-ground black pepper
1/2 cup olive oil
1 pound sliced smoked ham, such as Black Forest, cut into matchstick strips
1/2 pound sliced Gruyère cheese, cut into matchstick strips
3/4 cup chopped fresh parsley