In the skillet of bacon drippings, cook the mustard greens, green onion, and chopped mushroom stems until the greens are wilted. Stir in the cooking sherry and allow to cook until almost evaporated. Add the cream and 5 1/3 tablespoons (1/3 cup) cheese. Cook for a minute or two more on low heat until the cream has started to thicken. Stir in the bacon.
Place the filled caps on the baking sheet. Top the mushrooms with the bread crumbs and remaining cheese. Broil for 2 minutes until golden. Serve immediately.