Place the rinsed pinto beans into the slow cooker. I used my large crock pot. If you have the option to use a large, or small crock pot, use the large.
Fill your slow cooker 3/4 full with water, about 8-10 cups of water (it might vary depending on your particular slow cooker).
Sprinkle the crushed bouillon and garlic salt over the water.
Place the sweet onion into the slow cooker.
Cook on high for 6-8 hours or on low for 9-10 hours.Stir occasionally. If you need to add more water during the cooking process, be sure to do that.
Once the beans are soft, use a slotted spoon to transfer the beans to a large bowl. Keep the bean juice. Discard the onion.
Add 1/2 cup butter, onion powder, cumin (if using) and 1/2 cup of the reserved bean juice to the beans.
Using a hand blender, or a potato masher, mash the beans until they reach your desired consistency. Add more bean juice to make them less thick if you'd like. I added about 1 1/2 cups of bean juice total. The beans will thicken and set with time so keep that in mind.