Add potatoes, butter, pesto, water, garlic and spices to slow cooker. Cook on HIGH for 3-4 hours, or on LOW for 7-8 hours OR until potatoes are very tender.*
Add milk and sour cream, and mash with a potato masher or beat with an electric mixer (recommended) just until blended - don't overwork potatoes or they might become gummy.
Add additional milk as needed to reach desired consistency. Taste and season with additional salt and pepper if desired.
Notes
*Potatoes will require longer cooking time if you have an older slower cooker. *Cooking time will vary depending on potato size.