Preheat the oven to 375 degrees F. Remove the stems from the mushrooms, finely chop, and set aside. Set the caps, holes downwards, on a baking sheet, rub the mushrooms with a little olive oil, and bake 10 minutes to shrink slightly.
Meanwhile, melt the butter in a saute pan and gently fry the shallot until slightly soft, about 3 minutes. Add the chopped mushroom stems until soft, about 5 minutes more. Add the garlic, thyme, and pepperoncino one minute. Stir through all but a tablespoon of the bread crumbs. Taste, and season the mixture. Mix the parsley and remaining breadcrumbs together in a bowl.
Pull the mushrooms from the oven and turn them hole-side up. Season the mushrooms with salt and freshly ground black pepper. Divide the cheese evenly among the mushrooms, pile on the filling, and then scatter the bread crumb-parsley mixture evenly over top. Drizzle with olive oil, and bake until the tops are golden and the filling is very hot, about 20 minutes
Serve with watercress or pea sprouts on the side and a generous swirl of balsamic reduction.