Peel and chop the russet potatoes into equal-sized chunks.
Coat the slow cooker with butter or cooking spray to prevent sticking.
Add potatoes, garlic (if using), salt, and half of the milk/cream mixture.
Cover and cook on low for 4-5 hours or high for 2-3 hours, until fork-tender.
Add butter and remaining milk/cream, then mash until smooth and creamy.
Season with salt and black pepper to taste.
Keep warm in the crockpot until ready to serve.
Optional: Stir in cheese, roasted garlic, or fresh herbs for extra flavor.