Place the mushrooms in a medium glass bowl. Add the vinaigrette and toss gently. Allow to sit at least 10 minutes, until the mushrooms are wilted and impregnated with sauce. Check for seasoning and add additional salt and pepper, if necessary.
Place shallot, garlic, parsley, mustard, vinegar, salt and pepper in the bottom of a small mixing bowl. Stir to combine, then whisk in olive oil in a slow, steady stream. Alternately, place all ingredients in a clean 8-ounce jar. Cover and shake vigorously.