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

Melt the butter in a medium skillet over medium heat. Add the chopped mushroom stems and cook for 5 minutes, or until most of the moisture is gone. Add the onion, garlic, salt, and pepper, and 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, add the cream cheese, parmesan (reserve 1 tablespoon), parsley, and pecans (reserve 1 tablespoon). Stir until well combined.

Fill mushroom caps with the cheese filling, and evenly sprinkle the tops with the reserved parmesan and chopped pecans.

Bake for 20 to 25 minutes, or until the top is golden and the mushrooms are slightly soft. Garnish with extra parsley to serve.