Preheat the oven to 375 degrees F.

Coat a large saute pan with olive oil, add the pancetta and put the pan over medium-high heat. Cook until the pancetta starts to become brown and crispy. Add the onions and crushed red pepper, and season with salt. Cook until the onions are very soft and aromatic, about 5 minutes. Add the garlic and cook for another minute. Stir in the mushroom stems and rosemary and saute until they look very brown and wilted. Add the white wine and cook until the wine has evaporated. Remove from the heat and let cool.

In a large bowl, combine the pork, breadcrumbs, Parmesan and egg. Add the cooled mushroom mixture and combine well. Make a little tester patty and cook it to make sure the seasoning is correct. Season with salt if needed. If the filling seems a little dry, add 1/4 to 1/2 cup of water to really moisten things up.

Fill each mushroom cap generously with the filling. Arrange on a baking sheet and bake until the filling is cooked through, brown and crisp on top, 7 to 8 minutes. Transfer to serving platters and serve hot.