Grease the insert of a 6-quart (or larger) slow cooker. Add the potato chunks, chicken broth, butter, garlic and salt.
Cover and cook on HIGH for 3-4 hours or LOW for 6-7 hours.
Check the potatoes by piercing a few chunks with a fork. They should slide off with zero resistance. If they are still firm, cook for an additional 30 minutes.
Do not drain the liquid. The broth has reduced and contains starch and flavor that is essential for the texture.
Add the sour cream and hot milk directly into the pot with the potatoes and liquid.
Using a hand masher or ricer, mash the potatoes inside the pot until smooth and combined. Do not use an electric mixer or food processor, as this can make the texture gluey. Add the and parmesan cheese and mix until combined.
Taste and season with salt and pepper as needed. (If they are too thick for your liking, splash in a little warm milk). Garnish with fresh chives or parsley and serve warm.
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Notes
Best Potatoes: I highly recommend Yukon Gold (or yellow) potatoes for this recipe. Their naturally buttery texture creates the best mash, and you can leave the skins on if you like! If using Russets, make sure to peel them as the skins are tough.
Do Not Drain: When the cooking time is up, do not drain the liquid! It is full of flavor and starch. It will incorporate perfectly into the mash.
Avoid Gluey Potatoes: Mash by hand using a potato masher or ricer. Do not use a food processor or electric mixer, as this will turn the potatoes into a gummy paste.
Keep Warm: You can keep these on the warm setting of your slow cooker for up to 4 hours. Perfect for holiday entertaining!
Storage: Leftovers can be stored in an airtight container in the refrigerator for up to 5 days. Reheat in the microwave or on the stovetop with a splash of milk.