Portobello mushrooms are a common grassland fungi grown throughout the world. They are older versions of white buttons, and have a large brown cap with visible gills.
Fresh mushrooms are dry and firm to the touch. Slimy and blotchy shrooms have gone too far. Refrigerate in a paper bag. A crisper is too moist. Do not wash until ready to eat.
If you brush them, do so carefully. Otherwise, they can be rinsed in a bowl. Trim longer stems before use. Eaten raw, the flavor is mild and meat-like. Cooking awakens the fragrance. Marinade them in soy sauce and grill them with rosemary and olive oil!