1.To make caramelised onion, heat oil on barbecue flat plate; cook onion, stirring, over low heat, about 10 minutes or until soft. Add sugar and vinegar; cook, stirring, about 10 minutes or until onion is caramelised. Cool.
2.Cook sausages on heated oiled barbecue flat plate until browned all over and cooked through. Cool then halve sausages lengthways.
3.Split bread lengthways; spread top with relish, spread base with caramelised onion then top with sausage halves and watercress.
4.Season to taste. Press bread top on firmly; cut sandwich into eight slices.
Cook a double amount of sausages the next time you have them for dinner, to use in this recipe the next day, or use any leftover cooked sausages from your barbecue. Caramelised onion can also be cooked in a large frying pan; it can be made a day ahead and kept, covered, in the refrigerator until required.