Scrub and cut the potatoes to about 2-inch pieces. Peeling is optional. I don't peel the gold potatoes or the red. I do peel russets.
Add the potatoes to the crock. Add the garlic, if using, and 1 teaspoon of the salt, if using.
Add water, just enough to cover the potatoes.
Place the lid on the slow cooker and cook on High for 3-4 hours, or low for 7-8 hours.
The potatoes are done when a fork will pierce the potatoes easily.
Carefully drain the potatoes and add them back to the crock, or to a large bowl.
Add butter, milk, and salt (I suggest starting out with half of the milk, then adding the rest if needed), and mash with a potato masher (or use a hand mixer).
Taste and adjust ingredients to your liking and mash to desired consistency.
Add optional cheese, if using. Stir or mash more.
Taste and adjust the butter/salt/milk as desired.
Serve immediately, or keep in the slow cooker on the Warm setting (covered), until time to serve.