In a heavy skillet sauté lightly salted onions and mushrooms in olive oil about 10 minutes, add garlic and continue simmering on low heat another 5 minutes or until onions are translucent and mushrooms are tender. Add a little wine or stock if sauté becomes dry. In a stockpot over medium heat, bring stock, wine, pepper and garlic powder to a low boil. Transfer mushroom sauté and juices to stock. Reduce heat to low and simmer for 45 minutes to an hour. Just before serving, raise temperature and adjust seasoning to taste. Toast bread in the oven under a low broiler until lightly browned, top each slice with ½ oz. mozzarella cheese and broil just until melted. To serve, place cheese toast on top of each bowl of soup, and sprinkle with remaining mozzarella, freshly grated Parmesan and black pepper. Serve immediately.