Preheat your oven to 325F. Reserve a large baking sheet.
Gently wipe down the large mushrooms with a damp paper towel, cleaning any dirt or debris away. Remove the mushroom stems from the caps and reserve. Place the caps in a large bowl.
Once all the caps are removed, toss them gently with the 3 tablespoons of dry sherry and 3 out of your four tablespoons of olive oil. Place aside.
Gently wipe down the shiitake mushrooms or the gourmet mushrooms of your choosing. Chop finely, along with the stems of the large mushrooms reserved from earlier, and place into a medium sized bowl.
Peel & mince your garlic cloves, and wash & chop the scallions. Reserve.
In a large skillet, heat your remaining 1 tablespoon olive oil, over medium-high heat. Once heated, add the chopped mushrooms and cook for 3 minutes, making sure not to burn them. Stir often. Add the scallions, garlic, and a generous pinch of kosher salt and pepper. Mix to combine. Cook 2 more minutes. Add the panko bread crumbs and stir so they are evenly distributed.
Next, add the mascarpone cheese and parsley. Mix thoroughly. Adjust salt & pepper to taste. Remove from the heat.
Arrange the large mushroom caps on the reserved baking sheet (about 12 of them).
Take a generous spoonful of stuffing and fill each mushroom cap to slightly overflowing. Give a light sprinkle of kosher salt over each of the mushrooms.
Bake for 40-45 minutes, or until the mushrooms are cooked through and the tops are golden and tender.