Slice mushrooms and saute in olive oil in small skillet over medium heat until just tender. Remove to a small plate and set aside.
Add spinach to skillet, over medium heat, and season with salt, onion, garlic. Stir and cook a few minutes.
Lightly butter a 9" pie plate.
Line pie plate with cooked spinach.
Top with mushrooms.
Drizzle melted butter over.
Top with cheese and pepper.
(Can refrigerate at this point, covered. Bring to room temp before baking, if made ahead.).
Bake uncovered at 350 for 20 minutes until sizzling.
