Preheat oven to 400°.
Pierce potatoes with a fork; bake at 400° for 1 hour or until tender. Cool. Peel potatoes; coarsely mash. Discard skins.
Lightly spoon flour into a dry measuring cup; level with a knife. Place flour in a large Dutch oven; gradually add milk, stirring with a whisk until blended. Cook over medium heat until thick and bubbly (about 8 minutes). Add mashed potatoes, 3/4 cup cheese, salt, and pepper, stirring until cheese melts. Remove from heat.
Stir in sour cream and 1/2 cup onions. Cook over low heat 10 minutes or until thoroughly heated (do not boil). Sprinkle each serving with cheese, onions, and bacon.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.