In the slow cooker, combine potatoes, water, garlic cloves, bay leaf, and 1 teaspoon salt. Cover and cook until potatoes are tender, 4-6 hours on low.
Drain potatoes, discarding garlic cloves and bay leaf. Then return potatoes to slow cooker. Mash potatoes thoroughly. Fold in melted butter, then buttermilk, and sour cream. Add more warmed buttermilk as needed to adjust consistency. Fold in the green onions and season with salt and pepper to taste. Serve.
Notes
The mashed potatoes can be held on warm setting for 1 to 2 hours before serving. Loosen with additional warm buttermilk as needed before serving.