Preheat oven to 350°
quart casserole dish.
Wipe mushrooms with a damp paper towel. Separate stems from caps. Slice stems and very large caps. Leave medium and small caps whole.
Melt butter in a saute pan, add mushrooms, and saute until mushrooms are nicely browned, but not limp. Drain spinach and squeeze as dry as possible. Mix spinach with salt, garlic, onion, and melted butter. Spread spinach mixture in prepared casserole dish. Sprinkle with 1/2 cup cheese.
Spoon mushrooms on top. Sprinkle with the rest of the cheese. (Casserole may be refrigerated at this point.)
Bake at 350° for 20-30 minutes or until cheese is nicely browned and bubbly. Garnish with sliced black olives.