Preheat your oven to 400°F (200°C). Grease a baking sheet lightly with olive oil. Remove the stems from the mushrooms, finely chop the stems, and set them aside. Arrange the mushroom caps on the greased baking sheet.

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

To the mixing bowl containing the cooked mushroom mixture, add the cream cheese, most of the parmesan (reserving 1 tablespoon), parsley, and most of the pecans (reserving 1 tablespoon). Stir thoroughly until all ingredients are well combined into a cohesive filling.

Fill each mushroom cap with the prepared cheese filling. Once filled, evenly sprinkle the tops of the mushrooms with the reserved parmesan cheese and chopped pecans.

Bake the stuffed mushrooms for 20 to 25 minutes, or until the tops are golden and the mushrooms are slightly soft. Remove them from the oven and garnish with extra parsley before serving. Enjoy your delicious stuffed mushrooms!