Carefully pull stems from 20 mushroom caps and set caps aside. Finely chop stems and 4 remaining mushrooms. In a medium skillet on medium, heat 2 tbsp oil. Add chopped mushrooms and cook, stirring occasionally, until liquid has evaporated, 3 to 4 minutes. Add shallots and cook, stirring occasionally, until shallots are tender, about 3 minutes. Transfer to a medium bowl and stir in 6 tbsp panko. Set aside to cool slightly.
Meanwhile, preheat oven to 350°F. Mist a large rimmed baking sheet with cooking spray and arrange mushroom caps on top, stem side up.
Into chopped mushroom mixture, stir egg, chives, oregano, parsley, salt and pepper. Spoon mixture into mushroom caps, slightly mounding over top. In a small bowl, combine remaining 1 tbsp oil and 3 tbsp panko, then use mixture to top mushrooms. Bake until mushrooms are tender and topping is browned, about 30 minutes.
Let cool slightly, then arrange mushrooms on a serving platter and garnish some with chopped chives, some with oregano leaves and some with parsley leaves. Serve warm.