Boil the potatoes in a large pot of water until soft (about 25 minutes).
Set aside 1/2 pound of the cooked potatoes.
Mash the remaining potatoes and set aside.
Heat the flour in a large saucepan or pot over medium heat, then slowly add the milk, whisking until well blended.
Continue to cook the milk over medium heat until thick and bubbly (about 8-10 minutes), stirring frequently.
Add the mashed potatoes, 3/4 cup cheese, salt and pepper and stir until the cheese is completely melted.
Remove the soup from heat and stir in the sour cream and 1/2 cup green onions. Add additional salt to get desired taste.
Return the soup to medium heat and continue to cook until heated through (about 10 minutes). Do not let the soup boil.
Cut the reserved potatoes into small cubes and divide the soup into serving bowls.
Top each bowl with a few potatoes, cheese, green onions and bacon bits to serve.