Stack the other 7 sheets one by one on top of the first, brushing each with the oil-butter mixture. Pour in the spinach mixture and spread evenly. Brush the phyllo with the oil-butter mixture. Top with remaining 8 phyllo sheets, brushing each with the oi-butter mixture as you stack them. Tuck the overhanging edges around the inside of the baking pan to seal in the filling Bake until golden, about 45 minutes.
Chop stems and set aside.
Preheat oven to 450 Saute the onions and garlic in the oil on medium heat for about 8 minutes. Stir in the chopped mushroom stems and saute for 2 more minutes. Add basil, parsley, bread crumbs, cheese, salt and pepper. Cook for 2 - 3 minutes, stirring constantly, and set aside. Combine sherry and soya sauce in a bowl.