Place the potatoes, broth, garlic and butter into a 6qt slow cooker. Season with salt; mix together and cover with lid. Cook on HIGH setting for 4 hours or LOW setting for 7-8 hours, until very tender.
Mash potatoes with a potato masher or electric beater. Stir the sour cream and milk through until well combined and creamy (add more milk if desired). Stir through the parmesan cheese.
Keep on low or warm setting until ready to serve.
Notes
Best Potatoes: Yukon Gold potatoes are my top pick for that buttery, creamy texture, but Russets work great too.
Skin On or Off? Totally up to you! Leave the peels on if you want a more rustic, chunky mash.
Consistency Fix: If the potatoes dry out a little while sitting on the 'warm' setting, just stir in a splash of extra hot milk to bring them back to life.
Storage: Keep leftovers in an airtight container in the fridge for up to 4 days. (Just don't leave them out on the counter for more than 2 hours!)
High or Low setting which is best? Honestly this is completely up to the time you have, low and slow will not affect the taste as high and fast.