If you followed the brainy gourmet advice and kept only the stock in the purest form possible then yesterday's chicken soup can become today's tomato soup. Why do I say chicken soup when we are talking about and working with stock?
I say it because the original stock became chicken soup by adding carrots and onion, leaving out any pasta or rice which can be added to individual bowls. But, even if you do or did... let's say by accident or by choice added the pasta... you can always strain it out along with any vegetables to regain your stock which should now be called a broth.
Regardless, if you have chicken stock or a strained chicken broth, you should add one small can of tomato paste and cook for 20 min on med heat until you have a red rich tomato soup. Pasta or rice can be cooked on the side and added to individual bowls. Another side which my sweetie loves to have with tomato soup is a gooey grilled cheese sandwich. Now that's not only frugal but brainy delicious!