Heat a casserole over medium heat and add a tablespoon or two of olive oil along with the onions and garlic and a generous pinch or two of salt and allow to cook gently for 5 minutes or so.
Add the celery and carrots and sautè for another 5 minutes. (I find popping the lid on helps the vegetables to cook nicely as the heat gets trapped in the pan).
Next add the fennel, smoked paprika, and a generous amount of freshly ground pepper and cook for a minute, then add the tomato paste and cook for another minute.
Now add the ham hock, the can of tomatoes, crush the whole tomatoes in your hand before adding to the pot, along with the chicken stock (use some of the stock to rinse the can of tomatoes and add to the pot), the butter beans including their liquid, and the maple syrup and vinegar. Bring to the simmer, pop the lid on, and cook for about 2 hours or until the meat is tender and falling off the bone. When tender, remove the ham hock from the pot and transfer it to a plate. Shred the meat from the bone and return it to the soup. Discard the bone and excess fat and skin from the hock.
If you would like a thicker consistency continue to simmer the soup, lid off, for an additional 20 to 30 minutes or until thickened to your liking.
Taste and adjust seasoning. Stir through the reserved celery leaves and chopped parsley.
Ladle into bowls and serve with buttered toast and a wedge of lemon.