Place the cut onion, 2 cups rinsed pinto beans, 2 tsp salt, 1 tsp garlic powder, 1 tsp cumin, and 1½ tsp ground black pepper into a slow cooker.
Pour in 4 cups of water and stir to combine.
Cook on High for 8 hours, checking and stirring every 2 hours, adding just enough water as needed to keep beans barely covered.
Once the beans have cooked, mash the beans and onion with an immersion blender until as smooth (or chunky) as desired. Will thicken significantly when cool.
Serve hot, or let cool and refrigerate for up to 3 days or freeze for longer storage, up to 3 months.
For authentic refried beans (frijoles refritos), fry the mashed beans in an oiled skillet until a super thick paste forms.
Notes
If more than 9 cups total water is needed, use a lower heat setting next time.