Preheat your oven to 400°F (200°C) and lightly grease a baking sheet with olive oil.

Clean 20 baby bella mushrooms and remove the stems, placing the caps rounded side up on the greased baking sheet.

Melt 3 tablespoons of butter in a skillet over medium heat. Add the chopped mushroom stems, diced onion, and minced garlic, and sauté for 5–7 minutes until translucent.

Transfer the sautéed mixture to a mixing bowl and combine with cream cheese, grated parmesan, chopped pecans, and parsley until well mixed.

Fill each mushroom cap generously with the prepared filling, mounding slightly, and sprinkle with remaining parmesan and pecans.

Bake for 20–25 minutes until the tops are golden brown and the mushrooms soften slightly.

Remove from the oven, cool for a few minutes, and garnish with additional parsley before serving warm.