Preheat oven to 375 degrees F.
Whisk olive oil, balsamic vinegar, and salt and pepper together. Brush portobello with vinaigrette. Bake portobello on wire rack for 20 minutes.
Saute pancetta until crispy, then drain fat from skillet.
Add a little olive oil, onion, diced mushroom stem, thyme, rosemary, and saute until there is no more juice in skillet, and onions are getting some color.
Transfer this mixture into a bowl and add bread crumbs, parsley, 1/2 the cheese, a pinch of salt and pepper, to your taste. Mix well. Spoon onto mushroom cap pressing lightly. Put the remaining cheese on stuffing and bake for 16 to 20 minutes until cheese is golden.
This can also be broiled to desired color.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Cheryl Young, Marion's Restaurant, New York, NY