Melt butter in a large saucepan and cook the bell pepper, onion, celery, and garlic until tender, 5 to 8 minutes. Stir in the tomato paste, paprika, and Italian seasoning and cook an additional 3 minutes. Add 1 1/2 cups chicken stock or water, tomato juice, tomatoes, Worcestershire, salt, black, cayenne and white pepper. Bring the mixture to a boil, then reduce the heat and simmer for 8 to 10 minutes, stirring frequently.
In a small bowl, blend the cornstarch with 4 tablespoons water until smooth. Gradually add the cornstarch to the mixture, stirring constantly, until the sauce thickens. Sprinkle with parsley and serve.