Prepare the Mushrooms: Preheat your oven to 375°F (190°C). Clean the mushrooms and remove the stems. Lightly brush the caps with olive oil and season with salt. Arrange on a baking sheet.
Make the Filling: Mash the boiled potatoes with butter, sour cream, garlic powder, salt, and pepper until smooth. Stir in half of the cheese, bacon, and green onions.
Stuff the Mushrooms: Spoon the potato mixture into each mushroom cap, mounding slightly. Top with the remaining cheese and bacon bits.
Bake: Place the mushrooms in the oven and bake for 20-25 minutes or until the cheese is melted and bubbly.
Broil for Crispiness: Broil for 2-3 minutes for a golden, crispy topping.
Garnish and Serve: Sprinkle with chopped green onions and serve warm. Enjoy!