Stuffed mushroom recipes...

Chopped mushroom stems, garlic, shallot, parsley, bread crumbs, lemon zest, s&p. If you have any favorite herbs, add them in (thyme comes to mind). Moisten the mixture with a little cream, stock, or vermouth or sherry. Bake 15 minutes at 375.

Clean and simple; purchase champignon or other closed capped mushrooms with stems intact and no gills exposed. I hate portobella
which is just a marketing name for the common brown mushroom. Chop the stems and saute with garlic and parsley in butter. This is duxelles. Season with salt. Refill the caps and bake for about 15 minutes.

I like to use a combination of bread crumbs and, crab meat with a little salt and pepper , simple and tasty

You didn't say if it had to be meatless but if meat is ok here is a fantastic recipe
http://food52.com/recipes/3110-creamy-sausage-stuffed-mushrooms