Heat butter and oil in large, heavy pot over medium high heat.
Add chopped onion and cook, stirring occasionally, for 3-5 minutes or until onion is translucent.
Add potatoes and season with 1 teaspoon each of garlic salt and black pepper.
Cook, stirring occasionally, for about 5 minutes.
Add 8 cups water, bring to a boil, reduce heat and simmer until potatoes are tender - about 12 minutes, depending on size of potato chunks.
Remove from heat and blend until smooth with immersion blender.
Over low heat, stir in grated cheese until completely melted.
Taste and adjust seasonings. This is when I add a bit of cayenne pepper or Creole seasoning.
Stir in sour cream until completely blended.
Ladle into soup bowls, add desired toppings and serve.