Wash and peel the potatoes and cut into about 2-inch cubes. In a large stockpot, place the potatoes and 4 cups broth in and bring it to a boil. Boil potato until you can stick a fork thru them. This takes about 10-15 minutes. Be sure to reserve broth to add it back into the soup later on - it will reduce down to about 1-2 cups!
While potatoes cook, sauté butter, onions, and garlic in a medium sized frying pan on medium heat, until onions become translucent; about 5-8 minutes.
Blend the pistachios first using a powerful blender. Then, using a slotted spoon, place cooked potatoes in the blender, then pour in the onion mixture and reserved broth. Add in all additional ingredients (except the sour cream) and blend until smooth. Soup will be thick. Caution: when mixing, your blender will get hot, since the contents are hot!
Transfer this back into the pot you boiled off the potatoes in on the stove and let it come to a simmer. Stir in the additional broth 1/2 cup at a time until the soup reaches a desired consistency. Taste and add more salt and pepper if needed. Lastly, add in the sour cream and stir.
Enjoy warm and top with parsley and crushed pistachios!