In a medium skillet, brown seitan in the olive oil for about 3 minutes or until cooked. Reduce heat to low and add hot sauce. Stir well. Cook for another minute, then remove from heat and set aside.
In a large stock pot, add vegetable broth, cabbage, mushrooms, bamboo shoots, soy sauce, vinegar, red pepper flakes, and salt. Cover and bring to a low simmer for 15 minutes.
In a small bowl, whisk together cornstarch and water until smooth. Slowly stir into soup mixture. Continue to stir until soup has thickened.
Pour into serving bowls and top with green onions.