Heat the oil in a wok or deep frying pan. Using scissors, snip the sausages into quarters straight into the pan. Throw in the onion, and fry for a minute or two, stirring every now and then to brown everything. Spoon in the mustard and sugar, tip in the tomatoes and cook for a few minutes more, stirring occasionally, until the tomatoes are just beginning to split and the mixture is coated in the sweet mustard glaze.
Meanwhile, separate the lettuces into leaves, peel, stone and slice the avocado and halve and slice the cucumber. Mix them together and pile onto a platter. Spoon the hot sausage mixture on top. Add the vinegar and 1 tbsp water to the pan and stir it over the heat to dislodge all those tasty flavourings from the pan. Spoon the pan juices over the salad and serve with crusty bread.