Add the potatoes, diced onion, garlic, and 1 teaspoon of the salt to the crock.
Add just enough cool water to cover them.
Cook on High for 3-4 hours, or Low 6-8 hours, or until the potatoes are tender when pierced with a fork.
Carefully drain the potatoes and add them back into the crock. Remove the garlic and discard.
Add the butter, 2 tsp of remaining Kosher salt (or 1 1/4 tsp table salt), pepper, and onion powder. Mash with a potato masher and stir well.
Add the sour cream. Stir.
Add the green onion and chives and stir well.
If not as creamy as you'd like, add some more butter or milk, or more sour cream. Whatever you like.
Serve hot.