Use a 4-quart slow cooker. Pour the broth into your crockpot, and add butter and onion. Stir in frozen corn, garlic, butter, cayenne, and the crabmeat (make sure you pick through canned crab--shells often occur!) Cover and cook on low for 8 hours, high for 4 hours, or until the onion is cooked through and translucent.
If you'd like a thicker broth, pulse a few times with an immersible blender, or scoop out a cup or so and blend in a traditional stand blender, then stir it back inches Add half and half or cream. Stir well, and ladle into bowls or bread bowls, and garnish with avocado slices.