Preheat oven to 400 degrees F (200 degrees C). Butter a 9×13 inch baking dish with 2 tablespoons butter.
Place bacon in a large, deep skillet. Cook over medium high heat until evenly brown. Drain, crumble and set aside.
Place frozen spinach in a medium saucepan with 1/4 cup water. Bring water to a boil, then reduce heat to medium and cook spinach covered 10 minutes. Uncover and stir. Remove from heat and drain.
Remove stems from mushrooms. Arrange caps in the baking dish. Finely chop stems.
Melt 3 tablespoons butter in a medium saucepan over medium heat, and mix in onion and garlic. Cook 5 minutes, or until tender, then mix in bacon, spinach, chopped mushroom stems and heavy cream. Bring cream to a boil. Remove from heat and mix in Parmesan cheese, salt and pepper.
Stuff mushroom caps generously with the mixture. Drizzle with 2 tablespoons melted butter. Bake in the preheated oven 30 minutes until lightly browned.
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