Pierce potatoes with a fork; place potatoes on a baking sheet and bake in preheated oven for 1 hour or until tender. Cool. Peel potatoes; coarsely mash.
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. Divide between 8 bowls and top with cheese, green onions, and bacon.