Heat oven to 375 degrees F.
Sauté mushrooms in butter in large skillet over medium heat until softened and they lose their moisture, 3 to 5 minutes. Add spinach and garlic; cook until all moisture has evaporated, 3 to 5 minutes. Remove from heat.
Mix ricotta cheese, 6 tablespoons Parmesan cheese, eggs, salt and pepper into spinach mixture. Spoon into 2-quart baking dish. Mix bread crumbs with remaining Parmesan in a small bowl. Sprinkle over top of spinach mixture. (Can be made up to this point the morning of the dinner; cover and refrigerate. To serve, continue with Step 4.)
Bake in 375 degree F oven for 35 minutes or until bubbly and top is browned. Makes 12 servings.