The key to a really good, well flavoured French onion soup is to sweat the onions for a long time over a slow heat. They need at least 30 minutes before you add the stock. And believe me it makes a huge difference. You cannot make FO soup quickly.
You don't need mustard or soy sauce, just butter, onions and beef stock. Some people add a pinch of sugar to sweeten the onions and a bit of flour to thicken the soup.