In a covered medium saucepan, cook the cubed potatoes in enough boiling, lightly salted water to cover for 15 to 17 minutes, or until perfectly fork-tender.
Drain well.
Meanwhile, melt the butter in a large saucepan or Dutch oven over medium heat.
Whisk in the flour, salt, and pepper to form a smooth paste.
Cook for one minute, stirring constantly.
Pour in the milk all at once, whisking continuously until the soup base is smooth, slightly thickened, and bubbly.
Cook for 1 minute more.
Add the drained potatoes to the saucepan.
Using the back of a fork or a potato masher, gently press some of the potatoes against the side of the pan to create a rustic, chunky-creamy texture.
For the garnish, set aside 1/4 cup (960 ml) of the sour cream, 1/4 cup (960 ml) of the cheese, and 2 tablespoons of chives.
In a separate small bowl, temper the remaining sour cream by slowly whisking in about 1 cup (240 ml) of the hot soup. Return this tempered sour cream mixture to the saucepan.
Stir in the remaining cheese and 2 tablespoons (30 ml / 1 fl oz) of chives.
Continue to cook and stir over low heat until the cheese is completely melted and the soup is heated through. It's important not to let it boil after adding the sour cream.
Ladle the hot soup into bowls and top with the reserved sour cream, shredded cheese, and fresh chives before serving.
Don't skip tempering the sour cream!
Adding cold sour cream directly to the hot soup can cause it to curdle.
Mixing it with a little hot soup first ensures a silky-smooth finish.
