Combine the water, beans, ham hock, onion, carrot, celery, and thyme in a 4-6-quart slow cooker.
Cover and cook until the beans are tender, 5-6 hours on low. Total cooking time will depend on whether you like the soup to be very creamy (in which case you will want to cook about 8 hours on low) or whether you prefer the beans to be firmer to the bite (in this case, check after about 5 hours on low).
About twenty minutes before serving, remove the ham hock and the thyme stems and stir in the kale. Continue to cook for 15 minutes while you remove the meat from the ham bone. Then stir in the ham pieces along with the vinegar, salt and pepper.
Cook for another couple of minutes to thoroughly heat, and then ladle into bowls and top with a cheese toast, if desired.
To prepare the cheese toasts: Place the bread slices in an even layer on a cookie and bake for 10 minutes at 350 degrees or until crisp. If desired, you may brush with a little olive oil and rub with a garlic clove prior to baking. (You may complete this step in advance and store for several days in an air-tight container.) Just before serving the soup, sprinkle the bread with the cheese and broil, watching closely, until the cheese is melted and bubbly.