Place a large pot over medium heat with the olive oil. Add the onion, bell pepper, celery, carrots, garlic, thyme and curry powder to the pot. Season the vegetables with salt and pepper and sweat until tender, 5 to 6 minutes.
Add the sherry to the pot, if using, and cook for 1 minute to reduce. Add the chicken stock, potatoes and kale to the pot and bring the liquids up to a boil.
While the soup is coming up to a boil, in a medium mixing bowl combine the ground beef, nutmeg and some salt and pepper. Mix together to evenly incorporate the seasonings and then portion the beef into meatballs about 1 tablespoon in size. When the soup has reached a boil, add the meatballs, beans and mustard to the pot, season the broth with salt and pepper, and place a lid on the pot. Simmer the soup, gently stirring occasionally, until the meatballs and potatoes are cooked through, about 10 minutes.
Serve the soup garnished with Parmesan cheese, if using.
There are lots of different kinds of curry powders out there. I think this dish is best made with a garam masala type blend, which tends to be heavier in pepper, cinnamon, and clove; flavors which give this soup an amazing depth.