1Beat 1/2 cup of the milk and the flour with wire whisk until smooth; set aside. Heat oil in 4-quart Dutch oven over medium heat. Cook onion in oil about 2 minutes, stirring occasionally, until tender. Increase heat to high; stir in remaining 3 cups milk.
2Stir in potatoes, salt, black pepper and red pepper. Heat to boiling; reduce heat. Simmer uncovered 15 to 16 minutes, stirring frequently, until potatoes are tender.
3Beat in flour mixture with wire whisk. Cook about 2 minutes, stirring frequently, until thickened; remove from heat. Beat potato mixture with wire whisk until potatoes are slightly mashed. Stir in cheese, sour cream and green onions.