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1
Preheat oven to 375 degrees F (190 degrees C).
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2
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Transfer bacon slices to paper towels, reserving bacon drippings in the skillet. Crumble bacon when cooled.
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3
Remove stems from mushrooms and set stems aside. Place mushrooms, hollow sides up, on a baking sheet; brush the insides with melted butter.
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4
Chop mushroom stems; cook and stir chopped stems, black olives, green onion, and crumbled bacon in the bacon drippings over medium heat until mushroom stems release their liquid, about 5 minutes. Add garlic, salt, black pepper, and cayenne pepper; stir to coat.
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5
Tilt the skillet slightly; move vegetable-bacon mixture to one side, allowing the bacon drippings to pool to the other side. Whisk flour into bacon drippings until smooth and paste-like; slowly stir in milk until gravy is smooth. Mix vegetable-bacon mixture into the gravy. Add American cheese and Parmesan cheese; cook and stir until cheeses melt, about 5 minutes. Spoon filling into mushroom caps.
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6
Bake in the preheated oven until mushrooms are tender and filling is lightly browned, about 20 minutes. Remove mushrooms from baking sheet and cool on platter for 2 minutes before serving.
