Step 1: Prepare the Oven and Mushrooms

1. Preheat the oven to 400°F (200°C).
2. Lightly grease a baking sheet with olive oil.
3. Remove the stems from the mushrooms, finely chop the stems, and set them aside.
4. Arrange the mushroom caps on the prepared baking sheet.

Step 2: Cook the Mushroom Stems

1. In a medium skillet, melt the butter over medium heat.
2. Add the chopped mushroom stems and cook for about 5 minutes, or until most of the moisture has evaporated.
3. Stir in the onion, garlic, salt, and pepper.
4. Cook for an additional 1-2 minutes until the onion softens.
5. Transfer the mixture to a mixing bowl and let it cool slightly.

Step 3: Prepare the Cheese Filling

1. To the bowl with the mushroom mixture, add the cream cheese, Parmesan (reserving 1 tbsp), parsley, and pecans (reserving 1 tbsp).
2. Mix thoroughly until well combined.

Step 4: Stuff the Mushroom Caps

1. Fill each mushroom cap with the cheese mixture.
2. Sprinkle the tops with the reserved Parmesan cheese and pecans.

Step 5: Bake and Serve

1. Bake in the preheated oven for 20-25 minutes, or until the tops are golden and the mushrooms are slightly softened.
2. Remove from the oven and garnish with additional parsley.
3. Serve warm and enjoy your festive stuffed mushrooms!