Preheat the oven to 350 degrees F. Grease a sheet tray or line with parchment paper.
Clean the mushroom caps with a damp paper towel. Carefully break off the stems and, using the tip of a spoon, hollow out the center of the mushrooms.
Heat a large skillet over medium-high heat. Brown the sausage, breaking it up with the back of a wooden spoon. Transfer the cooked sausage to a paper towel-lined plate to cool.
Add the white chopped scallions to the skillet and cook until fragrant, about 2 minutes. Add the scallions to the sausage and cool.
When the sausage and scallion mixture is no longer hot, stir in the Parmesan, breadcrumbs, salt, pepper and softened cream cheese. The mixture should be very thick. Using a small spoon, fill each mushroom cap with a generous amount of the stuffing. Top with additional Parmesan if desired. Arrange the mushroom caps evenly on the prepared sheet tray.
Place in the oven and bake until the mushrooms are piping hot and liquid starts to form under the caps, 20 minutes. Garnish with the reserved green scallions.