Remove the stems from the portobello mushrooms or trim the ends of each stem of the white mushrooms. Arrange the mushrooms in a large baking dish. In a bowl whisk together the vinegar, mustard, salt and pepper. Gradually whisk in the olive oil until the vinaigrette is emulsified. Whisk in the garlic, rosemary and thyme. Pour the vinaigrette over the mushrooms and allow to marinate at room temperature for 20 to 30 minutes. Roast the mushrooms in a preheated 400 degree F. oven for about 20 minutes or until tender. Allow the mushrooms to cool to room temperature. Arrange greens on a platter. Cut mushrooms into 1/2-inch slices and fan on a serving platter. Drizzle with the reserved vinaigrette. Sprinkle the sliced mushrooms with the chopped parsley. Serve at room temperature.
Recipe courtesy of Gourmet Magazine