Toss the portabellas in the balsamic vinegar, 1/4 cup of olive oil, gray salt and pepper, to taste. Arrange the mushrooms on a baking sheet and roast in the oven for about 10 minutes. Remove from the oven and let cool to room temperature.
While the mushrooms are cooling mix together the bread crumbs, Parmesan, herbs and the remaining 1/4 cup olive oil.
Place 1 slice (2 if the mushrooms are large) of mozzarella in the cupped side of each mushroom. Distribute the bread and herb mixture evenly over the mushrooms and return to the oven to roast for 5 to 6 minutes or until the mozzarella is molten and the bread and herb topping a nice golden brown. Serve either hot or at room temperature and enjoy.