In a large pot combine the stock, onions, celery, butter, paprika and Worcestershire. Cook over medium heat for 30 minutes.
Add salt and beef base to taste. Add the caramel coloring as needed.
Preheat the broiler.
Fill a serving bowl with the soup and top with some croutons and cheese. Place under the broiler until the cheese melts and turns golden brown on top. Top with some parsley and serve.