This is absolutely one of those fabled grandma heritage recipes that is 1000x better the next day. Be sure to save some leftovers!
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.