First peel the shallots. Cut them in half and then into thin strips. Put into a large bowl. Mix vinegar, salt and sugar well, pour it over onions, mix well. Put a side plate over onions to fit inside of bowl, then put something heavy like a tin on top. Cover the bowl with cling film. Set aside for at least 2 hours. This is to pickle the shallots. They not need to go into fridge. After the time is up, drain the shallots well.
Cut the gherkin into small strips. Make the dressing by mixing it well together.
Peel the sausage and cut it into 2.5 cm (1inch) slices and then cut slices into strips. Quarter the apples, remove core, then cut into thin slices and then strips. Put sausage, gherkins and apples into a dish, cover with the dressing, gently mix.
Add the shallots, that have been draining into the bowl with the sausage, onions and gherkins, mix gently. Cover with cling film. Put into fridge, cool for about an hour. You can make this salad the day before eating. Taste and season. Add walnuts if you like.
