Add flour, 1 tablespoon at a time, mixing well after each addition. Stir in chicken broth, potatoes, corn, and corncob liquid. Bring to a boil, and cook for 5 minutes or until slightly thickened. Add Cajun seasoning, salt, and peppers. Reduce heat to a simmer, and cook for 30 minutes or until potatoes are soft. tir in half-and-half; simmer 5 minutes more. Garnish with green onion, if desired.