Preheat oven to 425°F.
Wash potatoes and cut slits in them with a sharp knife. Bake for 45-60 minutes.
Cut the potatoes in half and scoop out the flesh. Put to side.
Place large pot or Dutch oven on stove over medium heat.
Add butter to the pot and melt completely.
Add onion to the pot. Stirring frequently, cook until onions are soft and translucent.
Add flour to the pot. Stir to combine.
Pour milk into the pot. Increase heat to medium high. Wisk ingredients together, stirring constantly until liquid begins to thicken. Turn temperature down to low.
Spoon potato into the pot stirring to combine. Add shredded cheese and stir until cheese melts into soup.
If soup is too thin, mix cornstarch and water together, add to pot. Raise heat to medium high, stirring constantly until liquid thickens to desired consistency.
Ladle into bowls and garnish with cheese, bacon, and chives.