In a large saucepan, whisk flour and milk until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in the potatoes, 1/3 cup cheese, salt and pepper. Cook over medium heat for 2-3 minutes or until cheese is melted., Remove from the heat. Stir in sour cream and 1/4 cup onions until blended. Cover; cook over medium heat for 10-12 minutes or until heated through (do not boil). Garnish with remaining cheese and onions and, if desired, bacon.