Preheat the oven to 350 degrees F (175 degrees C). Spray a baking sheet with cooking spray. Clean mushrooms with a damp paper towel; carefully break off stems. Chop stems extremely fine, discarding the tough end of stems.

Heat oil in a large skillet over medium heat. Add garlic and chopped mushroom stems; fry until any moisture has evaporated, taking care not to burn garlic. Set aside to cool.

Stir in cream cheese, Parmesan cheese, black pepper, onion powder, and cayenne. The mixture will be very thick; use a teaspoon to fill each mushroom cap with a generous amount of stuffing. Arrange mushroom caps on the prepared cookie sheet.

Bake in the preheated oven until the mushrooms are piping hot, about 20 minutes.