Heat olive oil over medium heat in a stock pot. Cook onion until it begins to soften, about 5 minutes. Add garlic and cubed pumpkin and cook for another 5 minutes. Stir in the beef stock, tomatoes, salt and pepper. Bring to the boil, then turn heat to low and simmer until vegetables are tender, about 15 minutes.
While stew is simmering, bring a pot of salted water to the boil. Add the kale and simmer until softened, about 3 minutes. Drain into a colander.
After stew has cooked for 15 minutes, add the kale, ham and black beans; simmer for another 5 minutes. Stir in the sherry vinegar and sorrel; remove pot from the heat.
If unavailable, substitute in spinach and lemon zest or wild rocket.
Are smaller versions of the orange pumpkins you see around Halloween. They are sweeter and have more flavour. Don't use the larger ones, as they are grown to make jack-o-lanterns and have no flavour. If unavailable, use butternut squash.