Mince the green onion, parsley, tarragon, chervil, and celery leaves as finely as possible (you can do this in a food processor). Mix with the breadcrumbs and butter to make a fairly smooth paste. Season as desired with salt, pepper, Tabasco, and Herbsaint or Pernod.
Preheat your broiler. Trim the stem off each mushroom (you can mince these up and mix them into the filling). In a wide skillet, heat the olive oil on a medium flame. Lightly salt the gill side of each mushroom and sauté them gill side down until nicely browned, about 7 minutes. Remove from skillet and spread the filling onto the topside of each mushroom.
Place the mushrooms on a baking sheet and broil until the filling is bubbling and browned, 5 – 10 minutes, watching carefully to make sure they don't burn. Serve hot, garnished with fresh chopped parsley.