Mushrooms Stuffed with Breadcrumbs

2 lbs. fresh mushrooms
1/2 cup butter
1 medium onion or shallot, diced
garlic salt and pepper to taste
5 tablespoons bread crumbs

Wash and stem mushrooms, chop stems very fine
Melt butter, add stems and onions and simmer ten minutes
Add garlic salt, pepper and crumbs
Fill each cap with mixture, bake at 350 degrees 15 minutes