Spray the slow cooker insert with nonstick cooking spray. Add potatoes, garlic, salt, pepper, and 1 1/2 cups of the milk to the slow cooker. Stir to combine. Cook until the potatoes are tender on the HIGH setting for 4 hours.
Once the potatoes are tender, add the remaining 2 cups of milk and butter to the slow cooker and mash the potatoes with a potato masher until creamy.
Transfer the potatoes to a serving dish and top with optional toppings.
Notes
MAKE AHEAD INSTRUCTIONS:Cook according to instructions and then cool completely. Store in the refrigerator for up to 7 days. Reheat in the microwave or transfer to a baking dish and warm in a 350ºF oven until warmed throughout, about 30 minutes.