Cook the onions over medium heat in the oil until brown. Add the beef stock, onion soup mix and thyme and boil 15 minutes. Season to taste with salt and pepper.
Place the toasted bread croutons in individual onion soup bowls or large oven proof casserole. Ladle the soup over and sprinkle with the cheese.
Bake on the top rack of the oven at 375 degrees for 10 to 15 minutes or until brown. Serve hot.