Preheat oven to 400 degrees and spray a baking sheet with cooking spray.
Remove the stems from the mushrooms and finely chop them. (I cut off and discard the very end of the stem.)
Place mushroom caps hollow side up on prepared baking sheet.
Add olive oil to a nonstick pan and place over medium heat. Add sausage, mushroom stems, and onion. Cook, using a wooden spoon to break sausage into small pieces, until sausage is cooked through.
Add garlic and cook 1 minute.
Remove pan from heat and let mixture cool.
Stir in mayonnaise, stuffing mix, pepper, and cheese.
Place a heaping spoonful of filling into each mushroom cap.
Bake for 20 minutes.
Sprinkle with chopped fresh parsley
Notes
Tip: Unless the mushrooms are really dirty, don't rinse them. Just wipe them with a damp paper towel.