Preheat your oven to 350°F and line a rimmed baking sheet with parchment paper.

Clean the mushrooms gently with a damp paper towel, then remove the stems by giving them a gentle twist. Set the mushroom caps aside on your prepared baking sheet, gill-side up.

Finely chop the removed stems and set aside.

Heat a skillet over medium heat and add the butter.

Once melted, add the chopped mushroom stems and black pepper. Cook, stirring occasionally, until most of the moisture evaporates from the mushrooms (about 3 minutes).

Add the minced garlic and salt, cooking for another 30 seconds while stirring constantly. Remove from heat and let this mixture cool completely.

In a mixing bowl, combine the softened cream cheese, most of the Parmesan (save a few tablespoons for topping), onion powder, thyme, cayenne, and fresh parsley.

Add the cooled mushroom stem mixture and stir everything together until well combined.

Using a tablespoon or small cookie scoop, fill each mushroom cap generously with the cream cheese mixture.

Sprinkle the tops with the reserved Parmesan cheese.

Bake in the preheated oven for 20 minutes, until the tops are golden brown and the mushrooms are tender.

Serve immediately while hot, garnished with extra fresh parsley if desired.