Saute chopped mushrooms and onion in butter until juice has evaporated. Remove from heat and let cool. Then add remaining ingredients. Evenly spread about 2 Tbs of mushroom mixture between two slice of lightly butter bread. Bake at 375 degrees for about 10-15 minutes - turning once.
Note: Make a double batch, and wrap the the filled sandwiches in waxed paper and foil for freezing. They will keep in the freezer for a couple months. They are easiest to cut into triangles when they are frozen, prior to baking. You can usually find the "Very Thin" bread at the Pepperidge Farm outlet store.