In a large pot, fry minced onion and garlic together in 1/4 cup of oil or just enough to lightly coat everything. When onions have softened some add the beef stock and tomato sauce and heat to boiling.
While the stock is heating, form the meatballs: mix ground pork and ground beef together and add the rest of the ingredients to the mix. Form small meatballs and set aside.
Add meatballs, cilantro and optional zuchini to broth and cover. Reduce heat and simmer for at least 30 minutes. The soup will gain character as the cooking time is lengthened -- at about an hour and a half the meatballs will start to degrade pleasantly.
Ten minutes before serving add the sprig of mint to the broth and remove prior to serving.