Preheat the oven to 400°F (200°C). Lightly coat a baking sheet with olive oil.

Carefully separate the stems from the mushroom caps. Finely chop the removed stems and reserve them. Position the empty mushroom caps on the prepared baking sheet.

Melt the butter in a medium skillet over medium heat. Add the finely chopped mushroom stems and sauté for approximately 5 minutes, continuing until most of their released moisture has cooked away.

Stir the diced onion, minced garlic, salt, and black pepper into the skillet. Continue cooking for another 1–2 minutes, or until the onion begins to soften.

Transfer the cooked mushroom mixture to a mixing bowl and allow it to cool slightly.

Add the softened cream cheese, chopped parsley, and most of the grated Parmesan and pecans to the bowl, setting aside 1 tablespoon each of Parmesan and chopped pecans for the topping.

Mix thoroughly until the ingredients form an evenly combined, cohesive filling.

Spoon the prepared cheese mixture into each mushroom cap, dividing the filling among all 20 mushrooms.

Scatter the reserved 1 tablespoon of Parmesan and 1 tablespoon of chopped pecans evenly over the filled mushroom caps.

Bake for 20–25 minutes, until the mushrooms are slightly tender and the tops have developed a golden appearance.

Remove the stuffed mushrooms from the oven, garnish with additional fresh parsley, and serve.