The usual Pre-preparation of most varieties of mushroom is to remove any loose material with a dry cotton cloth or soft brush.. there is no need to do anything else, and washing begins the deterioration process. Those spongy membranes are the flavor but can be removed if the intention is to use a mushroom as a vessel such as a natural cup, or holder for fancy stuffings. Crab, dips and almost anything mashed will stay formed in a hollowed out cap.
Stuffed Portobello Mushrooms
Directions for stuffed mushrooms
1) Clean mushrooms (as described above); remove stems, leaving caps whole. Chop stems.
2) Sauté mushroom caps in 3 tablespoons of butter; set aside. Sauté chopped stems, green pepper, and chopped onion in remaining butter until onion is tender; stir in bread crumbs, salt, pepper, and cayenne pepper. Stir to mix well.
3) Stuff each mushroom cap with bread crumb mixture; sprinkle each stuffed mushroom with a little Parmesan cheese. Place stuffed mushrooms in a greased baking dish.
Bake at 325° for 15 minutes.
Recipe serves about 6 - 8.