Preheat barbecue on high. Cook onion on lightly oiled grill plate for 5 minutes, stirring, until soft. Add brown sugar and cook for another 2 minutes. Drizzle with vinegar and cook for another 2 minutes. Season to taste.
Meanwhile, cook sausages on open grill for 4 minutes each side, until cooked through and skin crisp.
Spoon caramelised onion into each bun, top with a sausage and serve with tomato sauce and mustard.