Put the french onion soup and 1 can of water in a large pot and add the shaved beef (I have also used very thin sliced steak, Bring to just barely a boil and reduce heat to simmer & cover for at least 45 minutes or until tender enough.
Meanwhile chop your veggies and saute them in the butter until transparent, set aside.
Butter your rolls and sprinkle a little garlic salt on them, broil until golden brown.
Remove lid from the beef and let it reduce if needed until the soup is a thick sauce. Spoon a generous amount onto each bun and top with the grilled veggies and then the cheese.
Put under the broiler again just until cheese melts. Enjoy!