Here's a pretty simple Roasted Portebello Sandwich recipe that's good and easy to cook, IMO:

Ingredients:

large french loaf
portebello mushroom caps
green onions
lettuce
aged swiss cheese
butter
oregano
dry garlic powder
fresh ground black pepper
salt
tabasco sauce
pepperoncinis

An iron skillet works fine for grilling.

Turn heat to high.
Usually you will need about 2-3 caps and 3-4 onions for a 6" sandwhich.
Chop mushroom caps and onions.
Add dash of salt, ground pepper, and garlic powder.
Saute with butter at high heat until the mushrooms are slightly browned.
Mushrooms may shrink a lot if over-cooked and can burn if there is not enough butter.

Turn heat down to med-low.
Prepare french loaf....cut in half.
Add butter and dash of oregano to eat half.
Heat bread beside mushrooms until slightly browned.
Bread should be soft and chewy, with outside crispy.
Flip halves over and repeat.

Everything is cooked.
Add mushrooms and onions to bread, add a stripe of tabasco.
Add swiss and lettuce.
Put pepperoncinis on side.
Enjoy.