1. In a small frying pan, toast the pine nuts over moderately low heat, stirring frequently, until golden brown, about 5 minutes. Or toast them in a 350° oven for 5 to 10 minutes.
2. In a large glass or stainless-steel bowl, whisk together the vinegar, mustard, salt, and pepper. Add the oil slowly, whisking.
3. Add the parsley, scallions, tomatoes, and mushrooms and toss. Top with the pine nuts.