Preheat the oven to 350 F. Gently clean the mushrooms with a damp cloth. Remove the stems and chop them finely.
Heat the oil (or water) in a skillet. Add the onion and cook until golden. Add the garlic and cook 1 minute more. Add the chopped mushroom stems, basil, oregano, and parsley, and cook 5 minutes, stirring, occasionally. Add the bread crumbs, petter, soy sauce, and sherry, and heat, stirring for 2 minutes. Add additional salt and pepper if needed.
Place the mushroom caps, open side up, in a lightly oiled baking dish. Fill each cap with the bread crumb mixture, forming a mound with your fingers.
Bake for 15 minutes and serve hot.