Place a stovetop-rated slow cooker liner over medium-low heat. (If your slow cooker liner isn’t made from a material that can be used on a stovetop, use a very large (12-inch) non-stick skillet.) Add the bacon in a single layer and cook, stirring occasionally until the bacon is crisp, about 20 minutes. If you're using the sliced bacon, remove 1/2 to use as part of your garnish. Leave 1/4 cup of the bacon drippings in the bottom of the pan and remove the rest. Raise the temperature to medium-high and add the onions, celery root, fennel (if you're using them) and 1 teaspoon salt. Stir to coat the vegetables. Add the garlic after about 10 minutes and continue cooking until the vegetables are softened and lightly browned, about 15 minutes. If you're using a skillet, transfer the vegetables to the slow cooker and stir in the chipotles, black beans, broth and bay leaves. Cover and cook on low for 8 hours or until the beans are tender.
At the end of the cooking time, remove and discard the bay leaves. Puree 3 cups of the soup in a blender until smooth, then stir back in along with the drained tomatoes and the hominy/pozole/corn. If the soup is too thick, add water to adjust the consistency, usually about 1 1/2 cups. Taste and adjust the seasonings with adobo sauce and additional salt. Cover and cook for another 30 minutes until all the flavors meld together.
Ladle into bowls, add a dollop of crema. Sprinkle on the diced avocado, green onions, cilantro and the reserved crispy bacon.
* Testing Notes: If there's a Mexican grocery in your neighborhood, you should be able to find cooked pozole corn there, otherwise use sweet corn.