Wash courgette and celery, peel onions. Cut all the vegetables into 6mm slices. Place in a bowl, sprinkle with salt, cover with plastic wrap and allow to stand overnight. Drain in a colander and wash under running water to remove the salt.
Place the sugar, vinegar and spices into a medium sized saucepan. Bring to the boil and simmer for 2 minutes, stirring until the sugar dissolves. Add the prepared vegetables,return to the boil, remove from heat and allow to stand for 2 hours.