Heat the remaining 2 tablespoons olive oil in a medium skillet over medium heat. Add the sausage, crumbling it with the back of a wooden spoon. Cook the sausage for 8 to 10 minutes, stirring frequently, until it’s completely browned. Add the chopped mushroom stems and cook for 3 more minutes. Stir in the scallions and garlic and cook for another 2 to 3 minutes, stirring occasionally. Add the panko crumbs, stirring to combine evenly with all the other ingredients. Finally, swirl in the mascarpone and continue cooking until the mascarpone has melted and made the sausage mixture creamy. Off the heat, stir in the Parmesan, parsley, and season with salt and pepper, to taste, Cool slightly.