Preheat the oven to 350°F. Place a wire rack on a rimmed baking sheet; set aside.

Set a medium skillet over medium heat and add the butter. When the butter is melted, add the chopped mushroom stems. Cook, stirring frequently for 3 minutes, until softened. Remove from heat and set aside.

In a medium bowl, combine the cream cheese, Parmesan cheese, pecans, parsley, onion powder, garlic powder, salt, and ground black pepper. Stir in the cooked mushroom stems. Adjust seasoning to taste.

Fill the mushroom caps with generous spoonfuls of filling and set them on the wire rack. Bake for 20 minutes, until heated through. Carefully set stuffed mushrooms into a paper towel for a few minutes to absorb any remaining excess moisture. Transfer to a serving plate and serve warm.