Add beans to slow cooker. Add room temperature water to cover by 2 inches. Let the beans soak overnight (8 to 12 hours).
Drain the beans and return to the slow cooker.
Heat the olive oil in a skillet over medium heat. Add the carrots, celery, onions and garlic. Cook, stirring often, until the onions are translucent, about 5 to 6 minutes. Scrape the mixture into the slow cooker.
Add bay leaves and chicken broth and give the mixture a stir.
Nestle the ham bone into the soup. Cover and cook on low for 10 hours or on high for 6 hours until the beans are soft. If you're planning to add additional ham (really only necessary if the ham bone used wasn't super meaty), add it a couple hours before cooking time ends.
Remove ham bone to a plate or dish (and any large ham pieces that may have fallen off during cooking). Shred ham into small pieces and return to the slow cooker.
If desired, use a potato masher to lightly mash the beans in the soup for a creamier consistency.
Taste and season with salt, if needed. Serve immediately or keep warm until ready to serve. This soup gets thicker the longer it rests. The leftovers are super thick and creamy (in a deliciously good way).
Notes
Broth:low-sodium broth is important - for us, this soup is salty enough using low-sodium broth and a hearty leftover ham bone. Different brands and types of ham will differ in saltiness. Full sodium broth might make the soup too salty. It's better to add salt at the end, if needed. Ham Bone:I prefer using a leftover ham bone from a spiral-sliced ham. In fact, I've made this soup several times without using the ham in another recipe first; I buy a ham just for this soup, cut off the meat and freeze it, and use the ham bone in the soup. I try and leave a good amount of meat on the bone so the soup has plenty of ham when it is finished cooking. Alternately, you can use 1 to 2 medium ham hocks in place of a large ham bone.Additional Ham:if the ham bone or ham hock(s) you are using don't have a lot of meat on them, you can add additional ham toward the end of cooking time (instructions included in the recipe).