Put dry pinto beans into slow cooker.
Fill crock-pot nearly to the top with water, about 3/4 of the way full.
Crush chicken bouillon cubes over the top of water, sprinkling evenly. Next, sprinkle garlic salt evenly over top. Drop the half medium sweet onion into slow cooker.
Cook on low until beans are soft, about 8-10 hours. (stir occasionally during cooking process. You may have to add more water as beans absorb water, so check on them periodically.)
Strain beans or pour off juice, saving the juice in a separate bowl.
Add 1/2 cup of butter, milk, and a small amount of juice that beans cooked in.
Use a hand blender to mash beans. Add milk or juice and blend to obtain desired consistency. Add more garlic salt as needed for added flavor. You want to leave beans a little runny as they will set up quite fast.