1) One day ahead: In a large bowl, combine the beef, pork, coriander, salt, pepper, cloves, nutmeg, and allspice. Mix until combined. Cover, and marinate in the refrigerator overnight.
2) On the day of serving: Grind the meat mixture in a meat grinder. Add the fatback and vinegar to the meat and mix well. Stuff the meat into sausage casing, if using, or else form the mixture into sausage patties. In a skillet set over medium-high heat, cook the boerewors until browned.
3) For the tomato bredie: Heat the oil in a large saute pan set over medium heat, and saute the onions until they're transparent. Add the pork and cook until brown. Add the tomatoes, potatoes, and chilli to taste. Cook, stirring occasionally, until the potatoes are tender and the tomato liquid is almost gone, about 20 mins.
3) Serve the boerewors with the Tomato Bredie.