The Food Lab: Better Home Cooking Through Science

Portobello mushrooms are mature cremini. Start by
trimming off the woody bottoms of the stems. Then,
cradling each mushroom in your hand, use a spoon to
remove the dark gills, which can discolor a dish and turn
it muddy. The mushrooms can now be scored for roasting
or grilling whole, or split in half and cut into thin slices for
sautéing.

Shiitake    Mushrooms

Shiitakes have very rubbery stems which should be
discarded. Use a paring knife to trim them off (removing
them by hand can end up tearing the caps). Slice the caps
into thin strips.