Melt 2 tablespoons butter in a large saucepan. Add shells and 1/4 cup onion and cook over medium heat until fragrant and lightly browned. Add chicken broth and simmer 10 minutes. Strain and set aside.
Stir in broth, sherry, thyme, bay leaf, and tomato paste. Simmer 15 minutes or until thickened.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)