Clean the mushrooms by wiping them with a damp cloth to remove any dirt or debris.
Cut off the stem of the mushroom at the base.
Place the mushroom cap on a cutting board, gills facing down.
Slice it into thin or thick pieces depending on your preference, working from one end to the other.
If desired, you can also cut the mushrooms into cubes or any other desired shape.
Note: Portobello mushrooms are quite delicate, so be careful not to apply too much pressure when cutting them.