Hot and Sour Soup with Tofu

-chestnuts, drained Heat oil in large saucepan over medium-high heat. Add pepper and onion; stir fry about 5 minutes. Add chicken broth, vegetable stock and soy sauce. Bring to boiling. Lower heat; simmer for about 5 minutes. Stir together vinegar, red pepper flakes, salt, pepper, cornstarch, water and sesame oil in a small bowl until smooth. Add to soup with fresh snow peas; cook for about 5 minutes or until thickened and bubbly. Add tofu, frozen snow peas if using, and water chestnuts. Gently heat through.