Preheat oven to 350°F.
Clean mushrooms with a damp paper towel. Carefully break off stems. Chop stems extremely fine, discarding tough end of stems.
Heat oil in a large skillet over medium heat. Add garlic and chopped mushroom stems to the skillet. Fry until any moisture has disappeared, taking care not to burn garlic. Set aside to cool.
Once cooled, stir in the cream cheese, bacon, parmesan cheese, black pepper, onion powder, and cayenne pepper. Mix together.
Fill each mushroom cap with a generous amount of stuffing. To make filling the mushrooms easier, use a piping bag with the tip cut off. Or, use a plastic baggie with the corner cut off.
Arrange the mushroom caps on a clean cast iron skillet.
Sprinkle the mushroom caps with shredded mozzarella cheese.
Bake for 20 minutes in the preheated oven, or until the mushrooms are piping hot and liquid starts to form under caps.
Serve immediately.