Preheat your oven to 400°F (200°C) and bake the potatoes for about 45 minutes or until tender.

Once baked, let the potatoes cool, then peel and dice them into small cubes.

In a large pot, melt butter over medium heat and sauté the chopped onion until translucent.

Sprinkle flour over the onions and stir to form a roux, cooking for about 2 minutes.

Gradually pour in chicken broth while whisking to avoid lumps.

Add milk and bring the mixture to a simmer, stirring frequently.

Fold in the diced potatoes and cook for an additional 10 minutes.

Stir in cheddar cheese and sour cream until melted and well combined.

Season with salt and pepper to taste.

Serve hot, garnished with chopped green onions.