Sift through beans, removing any stones and place in a 6 quart slow-cooker pot (or a large bowl). Add ~12 cups cold water and allow to soak overnight.
Drain off soaking water and rinse beans about three times.
Place beans in the slow cooker along with 1 onion, 4 cloves garlic, and 10 cups of water (about 2 inches above beans), cover, and cook on high for 5-7 hours or until beans are soft.
Ladle out and reserve 1 cup of liquid. Pour beans through a colander in the sink to drain.
Place beans back in the slow cooker (or in a large bowl (if you think you might scratch your pot with the hand blender). Sprinkle with 2 teaspoons salt. Puree the beans (as well as the onions and garlic) with an immersion hand blender or in batches in a regular blender. Add cooking liquid back to the pureed beans to reach desired thickness. Mash beans to your liking- chunky or smooth. (If using a regular blender, add enough liquid to get things moving.)
Serve or store in an airtight container in the fridge for up to 4 days. To freeze, place in plastic quart size freezer bags, press flat, and freeze up to 6 months.
*Recipe makes about 12 cups. Nutrition facts are for a 1/2 cup serving.