Combine the potatoes, broth, garlic, and salt in the slow cooker. Cover and cook on high for 4 hours, until the potatoes are tender (a knife should slide right into a potato cube with no resistance).
Use a potato masher to mash the potatoes right in the slow cooker. You can also use an electric mixer, but be careful not to overbeat them or they will start to become gluey and dense. If you want smoother potatoes, you can run the potatoes through a food mill or a ricer and return them to the slow cooker. Add the butter, milk, and cream, and stir to thoroughly combine.
If you like, you can re-cover the crockpot, set it to low, and hold them for up to an hour, stirring occasionally. Or leave them in the slow cooker on the warm setting for up to a few hours. Stir occasionally to prevent sticking and recombine everything.
Transfer to a large bowl to serve, or serve right out of the crockpot.
Notes
Variations and Mix-InsIf you want to give your potatoes some extra “oomph,” try stirring in any of the following!
1⁄2 cup soft goat cheese or shredded hard cheese, such as cheddar or fontina
1⁄4 cup freshly grated Parmesan cheese
1 to 2 tablespoons chopped fresh herbs, such as basil, oregano, thyme, tarragon, or chives