Remove stems from mushrooms, but do not discard. Using a small melon baller, scoop out insides of mushrooms, leaving at least half shell to allow more room for filling. Finely chop mushroom stems and centers.
Melt butter over medium heat in large skillet. Add onion and garlic; cook 1 minute. Stir in chicken, chopped mushrooms, Diamond Crystal(R) Kosher Salt, and pepper. Cook and stir 3 minutes longer. Blend in flour, cream and 3 tablespoons parsley. Cook and stir until bubbly and thickened. Remove from heat.
Spoon mixture evenly into mushroom caps. Place in lightly oiled, shallow baking dish. Bake 10 minutes. Top with remaining 2 tablespoons parsley and garnish with mozzarella or Parmesan cheese, if desired. Bake 5 to 10 minutes longer or until hot and cheese melts. Garnish with additional parsley, if desired.
